SOLVED: Microsoft 365 Email Error 451 4.7.0 Temporary server error. Please try again later. PRX4

We were trying to set up a phpBB bulletin board to send email using an existing Microsoft 365 Exchange email account but kept having it error out (Administration Control Panel > Maintenance tab > Error Log) with:

451 4.7.0 Temporary server error. Please try again later. PRX4

There were several superficial fixes we were able to find but none of them worked.  In the end we found two issues. You cannot programmatically send email through Microsoft 365 Exchange email server if:

1. You are using two factor authentication (2FA), which we do
2. “Authenticated SMTP” is not set correctly in Microsoft 365

Here is how SMTP Authenticated Email is Supposed to Flow:

The 2FA issue has no fix, so if you want to use two factor authentication, you can’t send email programatically (i.e. via phpBB)

The Authenticated Client SMTP issue is something you can fix by disabling and then enabling AUTHENTICATED SMTP:

1. Surf to portal.office.com, sign in, click the A icon for the ADMIN portal on the left rail
2. Expand Users > Active Users
3. Click on the user in question
4. Click the MAIL tab
5. Click the MANAGE MAIL APPS link
6. Uncheck it, click the SAVE button and wait 5 minutes (yes, you have to wait – if you just toggle the setting it does not seem to work)
7. Refresh the page and check AUTHENTICATED SMTP on, and click the SAVE button
8. Enjoy

For more details on Authenticated Client SMTP submissions see this Microsoft article.