separate from addresses with third party mail service
I got the same impression as you about the mailing service from the previous thread.
I have sent them messages saying that I have deleted all app passwords, and regenerated.
I have then used the Preview function in each of the three PowerMessenger installations to send separate messages, each with its request for a distinct From field, that I have clearly named per each domain.
Thus although the actual sign in name to the service remains the same, with three separate appwords (which was in any case happening) and three separate mailings arriving at them witnin a minute or so of each other from the three separate domains, hopefully they will either figure out for us what the problem is, or else amend things their end to make this possible.
I will let you know in due course what they say.