Today, one of my customers noticed, that the contact form on his site was not working anymore. Since nothing had changed for quite a while, I went to look, and it was actually so. A short check with the error returned from the SMTP mail library (PEAR_Mail) told me, the error was
550 Die verwendete Absenderadresse gehoert nicht zu Ihrem authentifizierten STRATO Paket.
For those of you who don't understand german, it translates roughly to "The used sender's address does not belong to your authenticated STRATO account." So I thought, no shit, it never has been, because I send the mails using the email address the user enters, so my customer can simply click "reply" in his mail program. This had never been a problem before, so why was it now. What complicated the matter even more, was that there is another form on the page, that also uses data entered by the user as the sender's address, and it worked perfectly well.
To keep things short: After tracing the tcp communication, I could not find any relevant difference. So, I tested sending a mail with a simple "This is a test" message. What can I say: It worked! Björn's an my guess is, that the STRATO mail server uses a spam filter and did not accept my mail due to its content. I removed some superfluous server data always submitted when sending a contact mail, and now things are working again. That error message, though, is really misleading, and I hope they change it to something more meaningful.
So, if anyone else has this problem, try checking your mails content. It might be "spammy" 