If you are using an O365 mailbox. Enable developers mode with assets. Go to Alias Domain and set Catchall Alias and Default From Alias to the same name as your outbound email server. Leave  Bounce Alias as bounce

Hi,

When you see the message “Invalid Operation, There was an error when trying to deliver your Email, please check your configuration” in Odoo, it usually means that the SMTP server is correctly set up but something is blocking the actual delivery of emails, such as authentication or security restrictions.

The first step is to test the outgoing mail server from Settings → Technical → Email → Outgoing Mail Servers. If the connection works, verify your SMTP credentials. Many providers (like Gmail or Office365) require the full email address as the username and sometimes an App Password instead of your normal password.

You also need to check the encryption settings. For port 587, enable TLS (STARTTLS). For port 465, use SSL/TLS. Port 25 is rarely used anymore because many providers block it. The “From” email address is also critical—some providers reject messages if Odoo tries to send using an address that does not match the SMTP account. Configuring a default “From Email” under Settings → General Settings → Email ensures consistency.

If emails are still failing, review the Email Queue under Settings → Technical → Emails. The error messages there, or in the Odoo log file, will usually indicate whether the problem is due to bad credentials, invalid sender address, or encryption issues. Finally, be aware of provider-specific rules: Gmail requires app passwords with 2FA, Office365 may require SMTP AUTH to be enabled, and Zoho only allows sending if the “From” matches the SMTP user.

Reference

* https://www.odoo.com/documentation/18.0/applications/general/email_communication/faq.html

Hope it helps

hm, same here with v18 - however test connection works OK 

Same issue here, not sure how to fix it.