After sending a mail to a known recipient, they receive an IMCEAEX undeliverability report with something like the following:
Diagnostic information for administrators:
Generating server: mailmNNN.d.ethz.ch
Remote Server returned '550 5.1.11 RESOLVER.ADR.ExRecipNotFound; Recipient not found by Exchange Legacy encapsulated email address lookup'
Outlook stores each recipient address by default in a local cache, also called AutoComplete list.
However, the address used from this list is outdated and invalid.
Delete the invalid entry from the AutoComplete list as follows:
Type the name of the recipient in the "To" field until it appears in the list and delete it by clicking on the X.