timeout after END-OF-MESSAGE

[Nikolaos Milas]

Hello,

From certain clients I see in the logs (true IP address / DNS name 
replaced):

May 14 18:36:57 mailserver postfix/submission/smtpd[21317]: timeout 
after END-OF-MESSAGE from workstation1.office.example.com [10.10.11.12]
May 14 18:36:57 mailserverpostfix/submission/smtpd[21317]: disconnect 
from workstation1.office.example.com [10.10.11.12]

These clients connect (using STARTTLS, port 587), authenticate (SASL 
auth) and submit mail successfully, but, obviously, do not disconnect 
normally and they timeout and are disconnected by the server (Postfix) 
after a timeout period which I see is 5 minutes.

Is this a known behavior of some clients (I suspect it may be Mac Mail) 
under certain circumstances?

Thanks,
Nick