> > Could it be some (older?) webmail clients that use pop3 instead of imap? > > I wouldn't expect a webmail client to hold a pop3 connection open > across multiple web requests. We have standard webmail clients > available for customer use, but they use IMAP. With the frequency > we're seeing this problem, I'd expect it's more likely to be something > newer or more commonly used.
Presuming you've been able to identify which users this is affecting I would suspect you could go back to those users and determine what clients they are connecting with and then interested parties (dovecot devs?) could perform further investigation in a lab or whatever to determine what is going on. Maybe the client(s) is/are just whacky or there is a bug somewhere. You can also track down the source IP addresses which may give you an idea as to the client as well. If it is a RIM subnet then you may be able to assume it's a blackberry. If the PTR record for the IP is webmail.somecompany.com then you can probably contact the company and discuss with them. Etc. Some companies may have a proxy or something that is attempting to hold the connections open for faster response times maybe geared for slow link connections. People do a lot of "interesting" things from time to time. I also didn't see you mention it but presumably you are on a relatively recent version of Dovecot considering you are examining the source code.
