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.
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 email@example.com – not too bad.
To proprietary mailserver users – demand that your vendor / sysadmin to fix it, or enjoy the problems.