There are several aspects to consider.
I’ve done several tests with different email accounts, and emails were always being sent properly.
The problem seems to be in the captcha. I think sometimes is difficult to introduce the correct captcha (you need to introduce exactly the same word, and it’s case sensitive), and if you fail several times (I think it was the point with Aidan), your account will be locked by security reasons and it needs that one admin manages the issue.
I’ve added a new captcha system into ‘Request new password’ and ‘Become a contributor’ forms, the Google’s reCaptcha. It’s pretty simple and intuitive, and I think it can helps us with this problem.
If it works properly, we can extend it to the whole system and replace the Mollom’s captcha.
If not, we’ll have to reopen this issue and maybe doing specific tests with different email’s providers.