[...]mails started getting rejected by some server [...] until i found a leap year bug that messed up weekday names [...]

Now that's a cool bug. I never expected servers to actually validate weekdays. Checking for one of the seven weekday names at the right position, yes, of course. But not actually matching the name to the date. On the other hand, there might have been some lazy spammers that simply always sent "Monday" or something like that.

Alexander