We have recently been getting dozens of reports of accounts receiving the following errors on a regular, reoccurring basis:

 

 

It seems most prominent with Windows Live Mail where it occurs every 15-30 minutes (especially on IMAP accounts with several GB of email), although Outlook 2013 is throwing Send/Receive errors as well in both IMAP & POP, but it is more transient and recovers a few seconds later.

 

For us the problem seemingly began with v13.1.5451, but it has gotten far more common since updating to v13.3.5535.

 

Increasing the Server Timeout in email clients doesn't resolve the problem, although it may be useful to note that Apple Mail, Thunderbird, and Outlook 2003 customers do not seem to ever experience this problem. It is apparently just Outlook 2013 and Windows Live Mail clients (not sure about Outlook 2007).

 

In the detailed logs there doesn't seem to be anything amiss. I've been trying to get some customers to enable Diagnostic Logging in their email clients so we can trap what may be causing the errors.

 

For reference this is a bare-metal server with Windows Server 2008R2 Enterprise, Dual Xeon X3450, 16GB RAM, and 15K SAS Drives, Dual Intel Gigabit NICs (82574L). This Mail Server only handles client connections (we have separate servers for Incoming & Outgoing Gateways that handle email delivery and all Spam checks). Although we support around 10K users & 35K-65K emails/day, the average number of connections at any given point in time is @ 400 (Avg CPU is 1%, Avg Memory is 1900 MB, Avg Handles is 2286  and Avg Threads of 71). We've been running Smartermail Enterprise for a decade now.