We have just installed ZCS NE 5.0.1 (split domain with our old email server) and are slowly migrating users across.
One particular user using IMAP/SMTP has been experiencing problems sending messages via SMTP. Some relevant lines of /var/log/zimbra.log read as follows:
Jan 16 11:46:36 zimbra postfix/smtpd: connect from unknown[188.8.131.52]
Jan 16 11:46:37 zimbra postfix/smtpd: setting up TLS connection from unknown[184.108.40.206]
Jan 16 11:46:39 zimbra postfix/smtpd: TLS connection established from unknown[220.127.116.11]: TLSv1 with cipher DES-CBC3-SHA (168/168 bits)
Jan 16 11:46:42 zimbra postfix/smtpd: C3EC2142A080: client=unknown[18.104.22.168], sasl_method=LOGIN, firstname.lastname@example.org
Jan 16 12:00:27 zimbra postfix/smtpd: warning: network_biopair_interop: error reading 922 bytes from the network: Connection reset by peer
Jan 16 12:00:27 zimbra postfix/smtpd: timeout after DATA from unknown[22.214.171.124]
Jan 16 12:00:27 zimbra postfix/smtpd: disconnect from unknown[126.96.36.199]
So it appears that the user's client connects and authenticates OK but after about 5-15 minutes the connection drops. Some of the affected messages have multi-MB attachments, but others not. The user also reports that he has similar problems uploading large attachments to an email being composed on webmail.
Our server is located in Sydney, Australia (100Mbit connection to Equinix) and the user is located in California. He has reported similar problems connecting to our previous email server, and also other email servers located in Australia on separate networks to ours.
It looks to me like there is a problem with his connection or ISP, but he reports that he is able to use other email servers in the USA without trouble (unfortunately he can't use these for the Zimbra email).
Does anyone have any ideas as to why this might be happening? Any suggestions for how to solve the problem, or where I should start looking?