fix Postfix TLS configuration
fix Postfix TLS configuration

Postfix could not connect to mail.hashbang.sh with the following message:

postfix/smtp[16969]: warning: mail.hashbang.sh[104.236.46.93]:25: invalid
TLS cipher grade: "ECDHE-ECDSA-AES256-GCM-SHA384....": aborting TLS
session

Outgoing e-mails were stuck in the mail queue with the status "Cannot
start TLS: handshake failure". However, OpenSSL client established a TLS
connection from the same client to the same server using STARTTLS just
fine.

The cause of this was a syntax error in /etc/postfix/main.cf -- the
smtp_tls_mandatory_ciphers parameter, at least in the Postfix version
that comes with Debian 9, expects a grade (high/medium/low etc), and
a list of specific cipher suites can be configured in the grade-specific
parameter (in this case this is tls_high_cipherlist).

This change has unblocked the mail queue.

hashbang/shell-server