I agree with Jamie, we send a plain text email but some email clients may interpret things trying to be helpful and in doing so they may be unhelpful sometimes.
But, reviewing the code, it seems that when generating new passwords it should be constrained to the characters in this web.config setting.
<add key="PasswordGeneratorChars" value="abcdefgijkmnopqrstwxyzABCDEFGHJKLMNPQRSTWXYZ23456789*$"/>
so I'm not sure how the @ symbol got in there unless you have changed that setting or are using an older version of mojoPortal.
Best,
Joe