question: Why is it still not possible to send files bigger than 5 or 6 mb through email?
comment: Unlike most internet services, email requires both the sender and the receiver to have a server, and the servers must cooperate. This means hundreds of kinds of email server must all work with each other, including some very old ones. So the standard is very slow to move forward, to avoid leaving the oldest 1% of servers behind.
passages: As an example, when Google's Gmail service increased its arbitrary limit to 25MB it warned that: ""you may not be able to send larger attachments to contacts who use other email services with smaller attachment limits"".
  
  Note that all these size limits are based, not on the original file size, but the MIME-encoded copy. The common Base64 encoding adds about 37% to the original file size, meaning that an original 20MB file could exceed a 25MB file attachment limit.
  
  A 10MB email size limit would require that the size of the attachment files is actually limited to about 7MB.
  
  Section::::Malware.
   Email messages may have one or more attachments, which are additional files that are appended to the email. Typical attachments include Microsoft Word documents, pdf documents and scanned images of paper documents. In principle there is no technical restriction on the size or number of attachments, but in practice email clients, servers and Internet service providers implement various limitations on the size of files, or complete email - typically to 25MB or less. Furthermore, due to technical reasons, attachment sizes as seen by these transport systems can differ to what the user sees, which can be confusing to senders when trying to assess whether they can safely send a file by email. Where larger files need to be shared, file hosting services of various sorts are available; and generally suggested.
   This is because of a number of potential limits: 
  
  BULLET::::1. Mail systems often arbitrarily limit the size their users are allowed to submit.
  
  BULLET::::2. A message will often pass through several mail transfer agents to reach the recipient. Each of these has to store the message before forwarding it on, and may therefore also impose size limits.
  
  BULLET::::3. The recipient mail system may reject incoming emails with attachments over a certain size.
  
  The result is that while large attachments may succeed internally within a company or organization, they may not when sending across the Internet.
   Email attachment
  
  An email attachment is a computer file sent along with an email message. One or more files can be attached to any email message, and be sent along with it to the recipient. This is typically used as a simple method to share documents and images. A paper clip icon is the standard indicator for an attachment in an email client.
  
  Section::::Current usage.
  
  Section::::Current usage.:Size limits.
  
  Email standards such as MIME don't specify any file size limits, but in practice email users will find that they can't successfully send very large files across the Internet.
   Although the difference in download time between plain text and mixed message mail (which can be a factor of ten or more) was of concern in the 1990s (when most users were accessing email servers through slow modems), on a modern connection the difference is negligible for most people, especially when compared to images, music files, or other common attachments.
  
  Section::::Security vulnerabilities.
   Originally Internet SMTP email was 7-bit ASCII text only. Text files were emailed by including them in the message body. In the mid 1980s text files could be grouped with UNIX tools such as bundle and shar (shell archive) and included in email message bodies, allowing them to be unpacked on remote UNIX systems with a single shell command.
   Unlike the Internet protocols used for the exchange of email, the format used for the storage of email has never been formally defined through the RFC standardization mechanism and has been entirely left to the developer of an email client.
  
  However, the POSIX standard defined a loose frame in conjunction with the mailx program.
   While IMAP remedies many of the shortcomings of POP, this inherently introduces additional complexity. Much of this complexity (e.g., multiple clients accessing the same mailbox at the same time) is compensated for by server-side workarounds such as Maildir or database backends.
   Attaching non-text files was first done in 1980 by manually encoding 8-bit files using Mary Ann Horton's uuencode, and later using BinHex or xxencode and pasting the resulting text into the body of the message. When the "Attachment" user interface first appeared on PCs in around 1985, it used the uuencode format for SMTP transmission, as did Microsoft Mail later.
  
  Modern email systems use the MIME standard, making email attachments more utilitarian and seamless. This was developed by Nathaniel Borenstein and collaborator Ned Freed; and the standard being officially released as RFC2045 in 1996.
   Email poses special challenges for preservation: email client software varies widely; there is no common structure for email messages; email often communicates sensitive information; individual email accounts may contain business and personal messages intermingled; and email may include attached documents in a variety of file formats. Email messages can also carry viruses or have spam content. While email transmission is standardized, there is no formal standard for the long-term preservation of email messages.
   There are also storage limits to individual Gmail messages. Initially, one message, including all attachments, could not be larger than 25 megabytes. This was changed in March 2017 to allow receiving an email of up to 50 megabytes, while the limit for sending an email staying at 25 megabytes. In order to send larger files, users can insert files from Google Drive into the message.
  
  Section::::Features.:Interface.
   BULLET::::2. Octet allows for the transfer of arbitrary raw 8-bit bytes, with the received file resulting byte-per-byte identical to the one sent. More correctly, if a host receives an octet file and then returns it, the returned file must be identical to the original.
  
  BULLET::::3. Mail transfer mode uses Netascii transfer, but the file is sent to an email recipient by specifying that recipient's email address as the file name. RFC 1350 declared this mode of transfer obsolete.
   iMail messages may have one or more attachments. Attachments serve the purpose of delivering binary or text files of unspecified size. In principle there is no technical intrinsic restriction in the I2P-Bote protocol limiting the size or number of attachments. In practice, however, the slow speeds, overheads and data volume due to redundancy limit the viable size of files or the size of an entire message.
  
  Section::::Features.:Email spoofing.
   BULLET::::- email messages are generally not encrypted.
  
  BULLET::::- email messages have to go through intermediate computers before reaching their destination, meaning it is relatively easy for others to intercept and read messages.
  
  BULLET::::- many Internet Service Providers (ISP) store copies of email messages on their mail servers before they are delivered. The backups of these can remain for up to several months on their server, despite deletion from the mailbox.
  
  BULLET::::- the "Received:"-fields and other information in the email can often identify the sender, preventing anonymous communication.
   BULLET::::- There may be privacy concerns, since messages that stay on the server at all times have more chances to be casually accessed by IT personnel, unless end-to-end encryption is used.
  
  Section::::Protocols.
  
  While popular protocols for retrieving mail include POP3 and IMAP4, sending mail is usually done using the SMTP protocol.
  
  Another important standard supported by most email clients is MIME, which is used to send binary file email attachments. Attachments are files that are not part of the email proper, but are sent with the email.
   Some web-based mailing lists recommend that all posts be made in plain-text, with 72 or 80 characters per line for all the above reasons, but also because they have a significant number of readers using text-based email clients such as Mutt. Some Microsoft email clients allow rich formatting using their proprietary Rich Text Format (RTF), but this should be avoided unless the recipient is guaranteed to have a compatible email client.
  
  Section::::Servers and client applications.
   The performance of MH is very much dependent on the file system and other features of the operating system that it runs on. Every read of a message will require starting a separate program, a directory scan in a large directory and a file open before the data can even be accessed. On the other hand, mailbox formats such as mbox which use a single file often require insertion or removal of data in the middle of the file when messages are read or altered and this can be a very slow procedure. In these cases MH will be much faster.
   BULLET::::- Bulk messaging: The flow of "peer-to-peer" MMS messaging involves several over-the-air transactions that become inefficient when MMS is used to send messages to large numbers of subscribers, as is typically the case for VASPs. For example, when one MMS message is submitted to a very large number of recipients, it is possible to receive a "delivery report" and "read-reply report" for each and every recipient. Future MMS specification work is likely to optimize and reduce the transactional overhead for the bulk-messaging case.
   To send binary files through certain systems (such as email) that do not allow all data values, they are often translated into a plain text representation (using, for example, Base64). Encoding the data has the disadvantage of increasing the file size during the transfer (for example, using Base64 will increase the file's size by approximately 30%), as well as requiring translation back into binary after receipt. The increased size may be countered by lower-level link compression, as the resulting text data will have about as much less entropy as it has increased size, so the actual data transferred in this scenario would likely be very close to the size of the original binary data. See Binary-to-text encoding for more on this subject.
   Each email provider has its own email sending limits that can be found on their websites. The limits may be daily, hourly, or sometimes even per minute. Exceeding the limits may result in blocking the email account for a time period determined by an email host.
  
  Section::::Factors decreasing email deliverability.:Problems with email content.
  
  BULLET::::- Spam words
   This does not work well with network mounted file systems, such as the Network File System (NFS), which is why traditionally Unix used additional "dot lock" files, which could be created atomically even over NFS.
  
  Because more than one message is stored in a single file, some form of file locking is needed to avoid the corruption that can result from two or more processes modifying the mailbox simultaneously. This could happen if a network email delivery program delivers a new message at the same time as a mail reader is deleting an existing message.
   BULLET::::- Files being transferred are of a non-sensitive nature, making encryption unnecessary,
  
  BULLET::::- Files being transferred are already encrypted at the file level or are passing over an encrypted VPN, making encryption redundant,
  
  BULLET::::- Available TLS or SSL encryption modes do not meet desired level of encryption. This is common with older FTPS clients or servers that may have been limited to 40-bit SSL due to previous United States high-encryption export laws.
  
  It may not be advantageous to use control channel encryption under the following scenarios:
   A problem with early versions is that mail was stored on the NetWare System volume. This was not a problem when messages were brief text files. Later it was possible, although rather complicated, to store received mail on another volume, but new mail had to remain on System. This caused problems with NetWare servers with a small System volume and larger ones for data storage, as users often kept many large messages in their new mail folders.
   However, there is a downside of e-mail: volume overload. Emails often have large unnecessary quantities of information which is non-job essential and/or spam. Filtering through this junk does require additional time. The time required to manage email can cause an information overload. With excess email, people may feel that they will miss information due to the sheer volume on content they receive. Some individuals may find this volume to be a barrier to swift responses to emails.
   Prior to the availability of 8BITMIME implementations, mail user agents employed several techniques to cope with the seven-bit limitation, such as binary-to-text encodings (including ones provided by MIME) and UTF-7. However, each of these workarounds inflates the required amount of data for transmission of non-ASCII text. Some non-ESMTP servers allowed the sending of 8-bit characters, but it is risky to send such data to a server whose 8-bit capabilities are unknown.
  
  Section::::Extended Simple Mail Transfer Protocol.:On-Demand Mail Relay.
  