I’ve had problems with proprietary mailservers where they put strange characters in the email address. Example: poor_guy/PITA/INDOFOOD@indofood.co.id

Mailman refused that email address straight away.
Not my loss

However, sometimes there’ll be (l)users complaining that it’s our mailserver that’s malfunctioning, and won’t accept anyone telling otherwise.
Doh….

btw - Lotus Notes can put REALLY weird email addresses as well, but it seems to be configurable.
Example, internally, my email address is seriously doesn’t conform to Internet standard - it shows my position in the company’s hierarchy. But externally, it’s been configured so it’s accessible as my.name@mycompany.com - not too bad.

To proprietary mailserver users - demand that your vendor / sysadmin to fix it, or enjoy the problems.