I've seen something similar a couple of times, and the reason was a misconfiguration of Outlook Professional on the clients side, so if you use such program double-check all your configurations.
Else, are you sure that the postfix settings are written in the right place and applied?
and check if your values are correctly applied.
postconf | grep smtp_data_ | less
If you see the new values have not been applied use the command
as the zimbra user to correctly set the values. Mind that this change doesn't survive to a Zimbra Upgrade, so re-apply them after one.
postconf -e smtp_data_done_timeout=800s smtp_data_init_timeout=240s smtp_data_xfer_timeout=800s
However, I agree with phoenix that many more informations would be needed to clearly identify the problem.
Cine - Community Manager - ZeXtras