My mailhosting provider has been testing dovecot 1.0 beta 3 as a proxy server and they asked the customers if they would like to test it out and report any issues. I am using Thunderbird 1.5 (IMAP) and have been with this mailhosting company for two months and have no issues at all with their current setup which uses Perdition as their proxy servers.
The problem(s) I am encountered have been reported here a few times previously over the last couple of years but in searching I have not found any concrete solutions. When I first tested it out there was a continuous problem of a failure in saving a copy of an email to the sent folder although the message was sent. One of the staff of the mailhosting company noted this exact problem as well and believed the issue was Dovecot announcing different IMAP capabilities than the real backend IMAP servers (via the CAPABILITY command). This was supposedly fixed and initially there were no issues with the first set of test emails. However, in short order, this problem re-emerged and there would be sporadic failures to copy a message to the sent folder. Another issue would be that every now and then, when clicking on a folder in the left-hand pane (and no other activity going on with Thunderbird like downloading a new mail) Thunderbird would hourglass for 10-15 seconds as if it were trying to open a connection and was having difficulty. The maximum number of connections setup in Thunderbird is the default 5 and I am advised that this is the correct setting with their servers. Reducing the maximum number of connections does not appear to change matters in terms of avoiding these problems.
Changing back to the current system (ie not using the test proxy server) both these issues disappeared. I have been in communication with the mailhosting provider as to my findings but it would be most appreciated if others here would perhaps have some insight into this matter.
Peter