Outgoing server error

So I’m trying to add a new email to my account and I get a server error that reads “Can’t connect to server. ([535#5.7.3 Authentication unsuccessful [MN2PR01CA0061.prod.exchangelabs.com 2025-08-14T00:26:32.992Z08DDDA1E51502A18]])”

Does anyone have any clue on how to resolve this? It’s been annoying me for days. Thanks