If you get the following error in your Postfix log file, and email clients just appear to be hanging (e.g. stuff in Outbox):
Aug 3 08:18:59 host postfix/submission/smtpd[1353178]: timeout after DATA (0 bytes) from unknown[x.y.115.60] Aug 3 08:18:59 host postfix/submission/smtpd[1353178]: disconnect from unknown[102.220.x.y] ehlo=2 starttls=1 auth=1 mail=1 rcpt=1 data=0/1 commands=6/7
You might be dealing with a bad network connection. Although the error is on the server, the actual problem is on the email client. Here are some options:
- Try another network
- Try install Windows Updates
- Try another router, or fiddle with the MTU of your network
Reference
https://serverfault.com/questions/625630/postfix-refuses-to-send-mail-to-some-recipients