Undeliverable to internal email address

One of our user's is having a problem sending emails to another internal user. They receive an undeliverable message saying "The e-mail address you entered couldn't be found." I have seen this happen before and fix it by deleting the target email address from Outlook's cache and then typing the email back in, then it will continue to send emails successfully. I have tried this and it works, however only temporarily. After deleting the cached email address it only takes about a week before the user starts to receive undeliverable messages again.

I'm not the best at explaining things, so here is an explanation written in a different way:

User A gets undeliverable message when sending to user B.
User A can send to anyone else in the organization without a problem.
Nobody else has problems sending messages to user B.
Deleting user B's email address from user A's outlook client and typing user B's email again will allow messages to be sent, but only temporarily.

If neither of these make sense to you let me know where I'm losing you and I'll do my best to clarify.

Thanks for your help.
banksovAsked:
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Neil RussellTechnical Development LeadCommented:
Is at some point User A "REPLYING" to an OLD email from User B that had the bad email address in it?
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PostmasterCommented:
Check for a "bad" address in the senders contacts folder.
This may be resolving to the address.
You can remove the cached entry (NK2 file) in the manner you mentioned.

Then send again using the GAL address.
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