Hi Bigdifference,
I can see how that’s disruptive, especially after things were working before. Since both email addresses are correct, ask the recipients to whitelist MYOB’s email sending domains. You can also try a simple one-line, plain-text email (no links, logos, or attachments) to each address and cc yourself to see if it goes through. If it still bounces with the same error code, it’s time for our team to check whether those addresses are blacklisted on our side, reach out to them directly and they’ll look into it.
Cheers,
Princess