Gmail bouncing messages - occasionally

:confused:
I've set-up a gmail account as to be used as a spam filter. It was described in another thread on these boards. (I've recently signed with a 3rd party exchange server utilizing a split-domain so that I can enjoy the benefits of enterprise e-mail. so, now you know why...)
The e-mail address is one that I've had for a long time - more than 10 years! Simply discarding it isn't an option.

For the most part, I'm getting messages with no issues. However, every day since setting up the gmail forwarding account - I get at least one person telling me they got a message bounced back to them. no rhyme or reason. No particular sender.

Here's the error they get with the bounce

Unable to deliver message to the following address(es).

<[email protected]>:
This user doesn't have a gmail.com account ([email protected])



Anybody else not getting messages? - oh, gee how would you know?..

well, anybody?