I’m using Outlook as part of Office 365 on all 7 of my Win10 PCs. I recently started having an email problem and I’m not sure what the cause is or what to do about it. Up until about a two months ago I never previously had this problem.

When I send an email to a new person/address (someone I have not sent to before) the message does not go through. About 1 second after I click the Send button I get the following in my inbox:

===============================================

From: System Administrator
To: my name
Subject: Undeliverable {subject line of original message}

Your message did not reach some or all of the intended recipients.

Subject: subject of original message}
Sent: 9/30/2020 10:32 AM

The following recipient(s) cannot be reached:

'Recipient’s name & email address' on 9/30/2020 10:32 AM
Server error: '555 5.5.2 Syntax error. p20sm2647743qtk.21 - gsmtp'

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I do not have this problem when I send a message to someone I’ve sent to before or when I reply to a message I’ve received from someone.

I do have this problem on all of my PCs and I seem to have it regardless of which of my email accounts I’m sending from. That makes me think that it’s an Outlook issue rather than an email server issue (but I could be wrong).

Perhaps there is a clue in the bottom line above that begins with “Server error:” since I don’t know what any of that means.

Any clues or suggestions greatly appreciated!

Peter