I have the same experiences with some of my customers. In each case, they are using either Microsoft Outlook or Microsoft Outlook Express.
They are different customers using a LAN infra-structure. The funny thing is that in the LAN, some of them can use the SMTP and some cannot after the Upgrade from 7.15 to 7.16 and later to Reloaded 7.51. It is strange.
The error generated from the Outlook or Outlook Express is:
Your server has unexpectedly terminated the connection. Possible causes for this include server problems, network problems, or a long period of inactivity. Subject 'Re: Various', Account: 'mail.domainname.com', Server: 'mail.domainname.com', Protocol: SMTP, Port: 25, Secure(SSL): No, Error Number: 0x800CCC0F
There is no error on the server log.
Any idea?