UPN, SMTP domain & mailflow for user and shared mailbox on-premise exchange

Question

Hello,

In my organization, I have internal Active Directory domain: company.local and SMTP domain: company.com
User mailboxes are created with UPN: userid@mathieu.company .local but shared mailboxes are created with UPN: userid@mathieu.company .com. Primary email address (SMTP) of both user mailboxes and shared mailboxes ends with @mathieu.company .com

So. below are my questions specific to on-premise exchange.

Q1: Is there any co-relation between UPN and smtp address (primary or additional smtp alias) as far as mailflow is concerned? Does mailflow for user and shared mailboxes depend on UPN value in anyway?

Q2: What are prerequisites in reference to UPN for mailflow to work for on-premise user and shared mailboxes?

Q3: Is this required that one of smtp alias of mailbox (user/shared) must match UPN value of user account for mailflow to work?

Q4: How does changing UPN (from company.local to company.com and vice versa) affect mailflow for user and shared mailboxes? I mean if I change UPN of shared mailbox from company.com to company.local, then will emails be delivered to shared mailbox?

Looking forward to answers and explanation specific to above mentioned questions.

Answer 1

UPN isnt related to mail flow at all.

Answer 2

Hi,
User-Principal-Name attribute:
This attribute contains the UPN that is an Internet-style login name for a user based on the Internet standard RFC 822. The UPN is shorter than the distinguished name and easier to remember. By convention, this should map to the user email name. The value set for this attribute is equal to the length of the user's ID and the domain name.

Microsoft's recommended best practices are to match UPN to primary SMTP address, but not a must.
Otherwise there might be issues when migration, (for Exchange online) or mis-understanding when O365-related Apps asking for your "address".

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