due to unknown circumstance, net/smtp often returns successfully before SMTP server has completely processed the mail, therefore introduce a short delay to make SMTP test case more reliable.
due to unknown circumstance, net/smtp often returns successfully before SMTP server has completely processed the mail, therefore introduce a short delay to make SMTP test case more reliable.

HouzuoGuo/laitos,HouzuoGuo/laitos,HouzuoGuo/laitos