I assume this is an IMAP mail server.
The app's treatment of html vs. plain text is determined by what the server returns as the mime type for this particular message part.
You are right - this is IMAP (Courier IMAP).
And I think I found true source of this problem - this mail don't have (mandatory for use of Content-Type:) MIME-Version: header.
So Courier return:
BODYSTRUCTURE ("text" "plain" NIL NIL NIL "base64" 1498 21 NIL NIL NIL) INTERNALDATE "22-Feb-2013 07:00:08 +0100")and AquaMail use it.
Looks like Thunderbird (and many other mail clients) not use BODYSTRUCTURE in strict way and use headers anyway.
Also not all IMAP servers parse this kind of error in message same way (as I found Dovecot will return text/html in BODYSTRUCTURE even without MIME-Version).
But if HTML have HTML header then AquaMail display HTML, probably using MIME guessing?
So I ask programmer to fix generated mail, but I send you logs too.
In logfile there are two log session.
First is wrongly formated HTML (displayed as text/plain).
BODYSTRUCTURE ("text" "plain" NIL NIL NIL "base64" 1498 21 NIL NIL NIL) INTERNALDATE "22-Feb-2013 07:00:08 +0100")Second is a bit fixed version - programmer add HTML header to the body - now it's displayed as HTML, only with wrong charset encoding (no meta http-equiv in body - this probably fix this issue).
BODYSTRUCTURE ("text" "plain" NIL NIL NIL "base64" 1792 24 NIL NIL NIL) INTERNALDATE "22-Feb-2013 09:25:27 +0100")Darek