misleading NDR 550 5.5.3 RESOLVER.ADR.RecipLimit; too many recipients exchange 2010

recently i faced a strange issue where some users are not able to to send e mail to number of users even though they have a big limit

Suppose you have a user with a recipient limit of 5000 and intermittently when he sends an e mail to a certain list of users he got an NDR similar to this

This message wasn't delivered to anyone because there are too many recipients. The limit is 50. This message has 216 recipients

but the user has a recipient limit of 5000, so what is special in this message

checking the message tracking i can see the same message failed also with ambiguous address, looking at the recipients in that message i could find an address which doesn't have an E mail and it displays as 'user name', so this looks like a bad address, Mail tips also shows that its invalid address

removing the address from the recipient list will solve the issue and bingo message is sent