I have some important scripts on my website at "1and1 Internet" that have been using PEAR mail under PHP 5.2. Since it's not my server, I have no control over what's installed.
They recently sent an email informing me that they now have PHP 5.4 available and will be removing earlier versions in a few months. So I switched my account config to try the scripts on 5.4 and found that they fail! The failure is on the require_once lines for Mail.php and Mail/mime.php, presumably because they don't exist or can't be found.
Looking at output from phpinfo(), it looks to me like the same --with-pear config switch is there in both versions, but on 5.4 I also see something to do with PEAR in a section about Phar, and I have no idea what Phar is!
Anyway, is there something more specific that I can try before attempting to contact support at 1and1? Fortunately I am able to return things to 5.2 until I get it all working!
First you need to check if pear is installed.
I assume you have shell access, so that would be simply:
$ pear
That would give you a list of commands if pear is installed.
Then you need to install the Mail package (probably as root...):
$ pear install Mail
And other packages you might need like Mail_Mime, etc.