I will update all horde packages next week and test this. Thank you.

Makes sense.  Added config option to IMP 5.  Also, if error occurs 
during the download of data, fallback to sending the message with 
links only.

Changes have been made in Git for this ticket:

Request #9099: Add config option to disable multipart/related conversions

http://git.horde.org/diff.php/imp/config/conf.xml?rt=horde-git&r1=f92994a77d09b7d2badebcf5d55404a74820b7ca&r2=d2f9d03d6c60cd28c6f841cd251a8ac7415a6621
http://git.horde.org/diff.php/imp/docs/CHANGES?rt=horde-git&r1=f92994a77d09b7d2badebcf5d55404a74820b7ca&r2=d2f9d03d6c60cd28c6f841cd251a8ac7415a6621
http://git.horde.org/diff.php/imp/lib/Compose.php?rt=horde-git&r1=01b53c377117e903316d321159f6a46e40b2c1cd&r2=d2f9d03d6c60cd28c6f841cd251a8ac7415a6621

This is a delicated issue, but I think this is very important for 
IMP.. I will try to explain using my terrible english (lol)..

The IMP uses by default multipart/alternative for images and this 
creates a "trouble" in networks where the server can't access internet 
(outgoing port 80) ... Now I'm working at a company where we have this 
problem, the webserver can't access internet...

Analyzing the code of IMP and trying to find a solution I've 
identified the following item:

"./imp/lib/Compose.php" Line 1081
if (empty($options['nofinal'])) {
        /* Any image links will be downloaded and appended to the
          * message body. */
          $textpart->addPart($this->_convertToMultipartRelated($htmlBody));
} else {
          $textpart->addPart($htmlBody);
}


As my server can't access internet and don't have a proxy server, IMP 
always returns "connect timed out!" when try to get images and doesn't 
send the message.

I propose that this be done by conf file, for example:

if ($conf['compose']['usemultipart']) {
        /* Any image links will be downloaded and appended to the
          * message body. */
         $textpart->addPart($this->_convertToMultipartRelated($htmlBody));
} else {
        $textpart->addPart($htmlBody);
}

Sorry if this already exists in IMP, I couldn't find and I'm sure that 
it is a good discussion.