Likely, other people on your SMTP server are sending SPAM emails. People have reported it and hotmail filters all emails coming from the server. People have switched their hosts because of this.
I registered an account to a yahoo.com email. Yahoo received it as Bulk mail. Here is the header from the email sent:
In phpBB admin configuration
Use SMTP Server for email IS SET TO NO *
Say yes if you want or have to send email via a named server instead of the local mail function
Test2 setup SMTP server, created a second account;.. Same thing
Validation email was received in BULK mail folder.
Test 3 .. In yahoo, I told it the other email was not spam... Re registered acounts using SMTP mail server and with it set to NO. both recieved into my inbox.
I'm going to send a normal email with my SMTP server and see where it goes
Sending normail messages through my smtp server went to my inbox
Trys editing the phpBB subject line to see if that makes a difference
be back soon