I just had the following problem with Horde 5.1.5 on a Plesk 11.5.30 server: Emails could be sent with the webmailer, the mail contents were all fine except that no attachment was sent. There were no error uploading attachments. They just weren’t there anymore on the receiving side.

At first I thought it might be a problem with qmail, so I checked /var/qmail/control/databytes. It contained a 0 meaning unlimited. So it was not the problem.
Then I checked in the sent folder, it containted the following message:

Date: Mon, 17 Feb 2014 10:26:05 +0000
Message-ID: <20140217102605.Horde.K04MY6UxjZMSwHKs7gDH8A2@webmail.server.de>
From: user@server.de
To: user@server2.de
Subject: attachment
Received: from p54AB010E.dip0.t-ipconnect.de (p54AB010E.dip0.t-ipconnect.de
[84.171.1.14]) by webmail.server.de (Horde Framework) with
HTTP; Mon, 17 Feb 2014 10:26:05 +0000
User-Agent: Internet Messaging Program (IMP) H5 (6.1.6)
Content-Type: text/plain; charset=UTF-8; format=flowed; DelSp=Yes
MIME-Version: 1.0
Content-Disposition: inline

So the attachment wasn’t to be seen there. So it was probably rather a problem with horde than qmail.

Then I checked the PHP setting to find out whether we might have hit a limit (although a 8K attachment should probably be under all set limits…). But both upload_max_filesize and post_max_size were set to a high value in php.ini.

So the only thing left was really Horde itself. Long story short, this ended up being a rights issue. The temporary directory used by Horde (/tmp/.horde) was owned by the apache user. Changing the owner to the Horde user did the trick:

chown -R horde_sysuser:horde_sysgroup /tmp/.horde/