The emails are probably blocked by the anti-spam of these services. Does the email address you sending from actually exists? For example, if the from address is mantis@example.com, then you should create such email account so that it exists for anti-spam agents that do check.

We're using Mantis for CouchSurfing.com, and we're having the same problem: http://bugs.couchsurfing.com/view.php?id=240

Now bugs are sent out from bugs@couchsurfing.com, which is directed to my email address. No improvement.

Should I try changing the message?
Aren't there any related bugs to this one?

is this still relevant in the latest version (1.1.1) ?

It is probably caused by spam preventing rules. I have the same problem with mantis sending emails to my account at gmail.com

And I've found something, which seems to be the solution:
http://mail.google.com/support/bin/answer.py?answer=10336

Yes. I definitely think this is a configuration issue, so no bug to fix.

As a general suggestion it is always advisable to use the SMTP method to send mails through an "official" mail server, being that your own or gmail or something.

For more help on configuration, please use the forums:

http://www.mantisbt.org/forums