Error 550 5.1.1 bad mailbox name

[r2edline]

Getting the following error message when trying to reply to a specific email address.  This person contacted me through a craigslist ad, I responded and we emailed back and forth several times, however randomly yesterday I got the following error message when trying to reply.

Error 550 5.1.1 bad mailbox name.  Open the message and Send to try again. 

I've tried creating a new message, restarting the app, nothing is working.  He has sent me multiple emails since but I can't reply to any of them.

Any idea why this might be happening?

[Kostya Vasilyev]

Well what if the address in his emails really is invalid?

Most common : dots at the end ("john.smith.@foo.com") or spaces ("john smith@foo.com")