can't send e-mail via outlook (ver.16.80) on mac mini running Sonoma 14.2.1 - error code 17898

To find out if it's system wide or user specific, try this...

Open System Preferences>Users & Groups, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

Does it work in the new account?

Are you using the old format for new Outlook format? My new format does not work but the old format does.

Good day! Thank you for posting to Microsoft Community. We are happy to help you.

Based on your description, I understand that you have a query "How do I resolve Error Code 17898 on Outlook for Mac 2016?". Before we proceed, we need more detailed information about the situation you are experiencing.

The possible causes for this error code includes Mac OLM files might turn unattainable if Outlook for Mac crashes unexpectedly, Due to Virus or malware infection, Because of interruption during Mac Outlook installation, Due to crashing of MS Exchange Server, Improper shutdown of Outlook application, Devoid of changing format you attempt to trade in OLM files to Windows Outlook.

For further investigation, please help us providing the following information:

1. Have you tried selecting the IMAP account, change the port number as 587, use SSL to connect after marking the checkbox? Refer to I can't send or receive messages with Outlook for Mac - Microsoft Support.
2. Have you tried removing the account and re-adding it in Outlook?
3. Have you tried inserting username and password again? To re-type username and password accurately, Unbolt Outlook application > From Tools menu option, pick Accounts > opt for email account, from left pane of window > Insert accurate username and password > Also confirm accuracy of Outgoing and Incoming server > For closing window, hit red button in upper-left corner of window.

https://answers.microsoft.com/en-us/outlook_com/forum/all/how-do-i-resolve-error-code-17898-on-outlook-for/904402ae-9020-48d4-a56e-814009de3582#:~:text=The%20possible%20causes%20for%20this,Outlook%20application%2C%20Devoid%20of%20changing