Office 365 Users unable to Send email to on premises Exchange User

When an Office 365 user sends an emails to an on premises Exchange mailbox user, below error is reported in the Non-Delivery Receipt (NDR).

550 5.1.10 RESOLVER.ADR.RecipientNotFound;

Recipient TestUser@domain.com not found by SMTP address lookup

If the OU in which the Test User account is in AD, is not synced with Office 365 / Azure AD by Azure AD Connect above error will be reported in the NDR. This is because Office 365 is not aware of the on premises mailbox / user's email address.

Change Azure AD Connect connection properties to include the OU. Wait for the next Azure AD Connect sync to complete. Then users in Office 365 should be able to email the on premises Exchange mailbox user.