Send email from exchange 2019 to o365 shared mailbox

Question

Hi! I have On-prem Exchange 2019 and O365, without hybrid scenarios deployed. I added domain to my on-prem exchange and created send connector to internet. If i send email to user in O365 all works fine, but if i send mail to shared mailbox i get receive error LED=550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup

As far i understand it's happened because i doesn't have hybrid, right?

Answer 1

The likely reason is that you don't have a local representation of the shared mailbox, i.e. you created it directly in Office 365. Thus the local server doesn't "know" about it existence, and since it's set as authoritative for any recipients in your domain, it generates the NDR. An easy fix is to create a user object and populate the targetaddress attribute to point to the shared mailbox address in ExO (shared@tenant .onmicrosoft.com). In addition, you might want to consider soft-matching it, if you are already using AAD Connect.