At the moment I playing around with Smfony2. One of the things I wanted to do is sending an email via form. Therefore of course I used Swift_Mailer component which worked very good in local dev environment. But after deploying to 1und1 hosted webspace I received error that neither localhost nor smtp.1und1.de could be connected to from the webspace.
After lots of research I found out that the mailserver hostname is mrvnet.kundenserver.de and then it worked.
I hope that this saves someone some hours or minutes.