User not listed in domino directory

Hi

I’ve recently noticed such error:

Unable to deliver message to xxx@xxx.com

User xxx@xxx.com not listed in domino directory.

However, user is not newly created. Nor was there any new changes made to user’s document. Also, previously emails from external can reach user. Other emails can also reach user.

It just happened to this particular mail. I noticed the problem is intermittent. Has anyone faced this before?

Can shed some light on this?

Thanks

Rusyinni