The environment would be my first guess if Webrick works and Mongrel doesn't.
Any chance that Mongrel is running with the Rails environment set to 'development?' By default, I think config/development.rb has this line:
config.action_mailer.raise_delivery_errors = false
So you wouldn't see delivery errors as exceptions.
And in test, ActionMailer doesn't use the MTA at all:
config.action_mailer.delivery_method = :test