i met the same problem

First, replacing classes class.phpmailer.php and class.smtp.php that are in mantis\core\phpmailer\ by these [url]http://phpmailer.worxware.com/index.php?pg=phpmailer[/url].

Then I made the following changes in class.phpmailer.php:

1 - I changed the following parameter:
public $SMTPSecure = 'ssl';

2 - I created the method:
getSmtp public function () (
return $ this-> smtp;
)

Second, in class email_api.php located in mantis\core\ i made the following changes

line 813

if (is_null ($ mail-> getSmtp ())) (

g_phpMailer_smtp $ = $ mail-> getSmtp ();

)

Third, in class config_inc.php located in mantis\ i made the following changes

$ g_administrator_email = 'myemail';

g_phpMailer_method = $ 2;

$ g_smtp_host = 'smtp.gmail.com: 465';

$ g_smtp_username = 'myusername';

$ g_smtp_password = 'mypassword';

instead of this:

2 - I created the method:
getSmtp public function () (
return $ this-> smtp;
)

Second, in class email_api.php located in mantis\core\ i made the following changes

line 813

if (is_null ($ mail-> getSmtp ())) (

g_phpMailer_smtp $ = $ mail-> getSmtp ();

)

you could do this:

in line 309 change

private $smtp = NULL;

to

public $smtp = NULL;

is more simple.

And I got around this by configuring postfix to mail through google's smtp servers. (ISP blocks port 25 and shut off the ISP's smtp servers, with a switch to Google Apps.)

I've just setup a Google Apps account. I setup the gmail account with ssl / port 465 and it didn't work. I changed to tls / 587 and it worked fine. Here is Google's documentation for reference:

http://mail.google.com/support/bin/answer.py?answer=13287

This worked fine on MantisBT v1.2.4.