Hostmonster messes up mail sent from vB

All of the emails coming from the board, which used to come from the webmaster email address set in the Admin CP before I moved my sites to HostMonster, now come from <myusername>@<blah>.hostmonster.com

The FAQ at HostMonster says that this is the result of improperly formed From: headers generated by vB. Problem is I don't know how to fix it. Here is specifically what their FAQ says:

This is because our servers require you (or your script) to use a properly formatted, valid From: header in your email. If the From: header is not formatted correctly, empty or invalid our system will change the from address to be <username>@<host##>.hostmonster.com. To stop this, you must change the script you are using to correctly use a valid From header.

Examples of headers that should work would be:
From: user@domain.com
From: <user@domain.com>
From: "name" <user@domain.com>

Examples of headers that will NOT work:
From: "user@domain.com"
From: user @ domain.com

Note that my webmaster email is set as opalcat@gmail.com with no quotes or anything, so it's not something I've messed up by putting it weird into the AdminCP.

[Marco van Herwaarden]

The From is set like: "vBulletin Community Forum" <webmaster@vbulletin.com>

Which should be fine according to their own FAQ. Let them tell you what the exact From header is that you are sending and why they change it.