So ... an update on this problem. I checked my facts again and it turns out that Thunderbird rather than dovecot support for large file attachments on IMAP servers is iffy. Here are a collection of URLs for those interested in reading further:

http://www.hawkwings.net/2005/12/30/thunderbird-imap-bugs/
http://bleedingedge.com.au/blog/archives/2005/11/speeding_thunderbird.html
http://bleedingedge.com.au/blog/archives/2005/11/making_thunderbird_fly.html
http://www.google.com/search?q=thunderbird+large+imap+bug&hl=en&lr=&start=20&sa=N <http://www.google.com/search?q=thunderbird+large+imap+bug&hl=en&lr=&start=20&sa=N>
http://blog.fastmail.fm/?p=527
http://www.desktoplinux.com/news/NS7484204546.html

Note that the fixes mention in these articles are only partial fixes ... at least in my case.

The bug is that while large email messages do arrive and get stored, moving them between folders using Thunderbird results in a nasty infinite loop where a substation chunk of the file gets copied over and over again into a Maildir/tmp directory, eating time and disk space. I was initially suspicious of dovecot because I kept getting dovecot errors when moving large files onto the IMAP server. Removing the qmail message size limit only solved part of the problem.

I can reproduce this bug using qmail as the MTA, dovecot as the IMAP server over Maildir directories, and Thunderbird as the email client.Using the dovecot "hardlinks" option solves the problem when moving messages between directories in the same account, but not between accounts or across machines. A close reading of the relevant websites shows that 1) people in the Thunderbird community are already painfully aware of this problem and 2) there really aren't any better alternative email clients. Here's hoping there's a Thunderbird fix soon.

Interesting, eh? Fred

Dan McGarry wrote:

Dominic Marks wrote:

On Tue, 19 Dec 2006 10:28:57 -0800
Fred Bauer <fred@fredbauer.com> wrote:

Simple question: I've discovered that dovecot's IMAP server imposes a 
~5Mb limit on email message size. After a day of searching I cannot
find the configuration parameter that governs this size. What is it?
Or is there no such parameter?

If there is I've never noticed it. Can you describe the behaviour you
are experiencing, perhaps it isn't dovecot at all.


Indeed. I find it hard to imagine how message size limits could be
usefully enforced by Dovecot.

Fred, perhaps you could look into size limits on the
SendMail/Postfix/Exim side of things, where they're usually enforced.

Alternatively, if you want to avoid downloading large files without
first viewing the headers, that option is available in a number of mail
client packages, such as Thunderbird and (IIRC) Evolution.

Good luck....