Originally Posted by SolidHost

I was trying to figure out what was causing the display of all email accounts that were setup in an email application (e.g. Outlook, The Bat, and such). Full headers would show those accounts under the following line:

X-PopBeforeSMTPSenders:

all email accounts that were previously setup in an email application would show up, so the result would look something like this:

X-PopBeforeSMTPSenders: admin@domain.com,info@domain.com,name@domain.com and so on.

I didn't had a single clue what was causing this to happen, so I've tried googling around and I have found few similar things, although they didn't help in solving this issue. I've asked around few people for an answer and got either "I don't know" or "I've never had that problem".

After couple of hours, the problem was fixed. The answer is so simple, you'll laugh - I know I did.

Login to WHM, choose Server Setup and then Tweak Settings. There is an option: "Include a list of Pop before SMTP senders in the X-PopBeforeSMTP header when relaying mail". Uncheck this and that's all there is to it.

Note: you need to have a root access in order to do this.

I posted this only because I couldn't find an answer to it on search engines - whole 83 results on google for the "X-PopBeforeSMTPSenders:", although none of them helps.

So, if you ever come across a problem of this kind, hopefully you won't have to spend hours solving it.