(below the error message)
One of our user (maybe more we don't know yet) can't send an email one a her contact (not one of our user but someone from another company) It bounced back. This external email recipient is not in her address book. Besides it's not a contact in Exchange. The autocomplete has been deleted, then we manually entered the email address of this recipient. But it still boucing back. The other users can send email to this recipient. But for a reason or another it doesn't work for her.
We also have this problem with external users from other company who a working in our office (those users have their own email address not related with us but we've created a external contact in Exchange otherwise it will bounce, but sometimes it doesn't work.
A X500 has been added for all users. But it doesn't solve the problem.
is it possible to just get rid of all X500 and the cache? I can't believe we still have to manually add X500 address in Exchange 2016 or Exchange online. X500 is used for emails between internal users I'm aware of it. But it's very annoying.
We already have this issue with some users (working for another office with their own emails for whom we've created contacts in Exchange 2016)
Remote Server returned '550 5.1.11 RESOLVER.ADR.ExRecipNotFound; Recipient not found by Exchange Legacy encapsulated email address lookup'