1.2.3 Domain Spoofing Prohibited

I have the latest K9 with 2 email accounts configured. Each have their own Sending Mail server setup correctly. Sending from one account works, but sending from the other account gives Failed to send some messages 1.2.3 Domain Spoofing Prohibited error.
How do I fix this?

Maybe you could add which server works and which not? Might be relevant if a specific mail provider has some special rules

Both are mail servers from hosting company but different domains.

That message is coming from the server you have configured as your “outgoing mail server”. Double check that the credentials are in alignment with the email address on the account that you are sending from.

Separately, when identifying the version of K9 you are using please be specific, e.g., 5.90 – see the “about” page for that detail – as “I have the latest K9” is relative and not very useful.