Mail.php and PHP version of Server

Recently our web-host upgraded our PHP version from 5.2.10 to 5.2.17. All of a sudden the email from shops stopped working (contact form, order emails etc.). After looking for solutions to no avail I found out that one of our older shops that still had OC version 1.5.1 running still worked despite the updated PHP.

Thinking it had something to do with the last version of OC, I did a fresh install of OC version 1.5.3.1 on a server that has PHP Version 5.3.19 running on it. Contact form still doesn't work.

I then tried uploading system\library\mail.php from v1.5.1 to the fresh installed 1.5.3.1. Eureka! Emails now work again! I tried the same for 1 of our 1.5.4 shops running on the other server with PHP version 5.2.17 on it and it works as well!

So my question is... Why is that?

I looked at both mail.php files and I see some subtle differences between them but my understanding of PHP is limited so I have no idea what could be causing the problem. Anybody have any clues as to why this is? Could it have anything to do with how our server (or PHP settings) is set up?

Nobody a clue?

I'm having the same issue.

Everything is working fine but the emails.

Is there any progress on the subject?

Thanks in advance.