You are making much of office365 settings, but to be sending mail you should be using the settings provided by the registrar. Microsoft 365 really does not like sending mail using a from address that is not one of theirs!

As you have chosen not to provide the registrar or the domain names you have truly limited the advice you can be given to the most generic. We know what you are telling us. In this case basically nothing other than you are using office365. Not even if it is a corpoate, business or personal subscription. All of which apparently have different reule to be followed. Such as https://docs.microsoft.com/en-us/microsoft-365/admin/email/change-email-address?view=o365-worldwide

Free live accounts actually provide the setting you should use on this link https://outlook.live.com/mail/0/options/mail/accounts so there is no need to guess what is correct for you. They tell you. I assume paid 365 accounts have similar settings similar link given the plethora of setting and requirement an administrator has access to.

This znet article would indicate that you can only do what you are apparently trying to do if your registrar is godaddy. https://www.zdnet.com/article/heres-how-office-365-users-can-get-a-custom-email-address-without-corporate-headaches/

Thank you for replying. I really did not know know the questions to ask nor what information you needed, given that I never have had to do this before. I have had Thunderbird for quite some time in connection with an sbcglobal.net domain-based email and had little problem with it over years of use. I would much like to be a geek, but I have trouble enough staying ahead as it is in those professional areas where I know a bunch of stuff.

Now I'm learning what's important for you to be able to answer more specifically:

1. GoDaddy is the registrar.

2. The domain is rickchristensen.com, which I secured through GoDaddy some time back and parked it there. I'm preparing to use it soon for some marketing purposes, so I guess it's to be business, but right now it's personal. GoDaddy never asked what I would use it for at the time nor what the two email addresses would be used for right now.

3. You wrote, "...you should be using the settings provided by the registrar." GoDaddy said nothing about this at all even when I told them I preferred client-managed email---Thunderbird specifically---over web mail and planned to use it. In the third paragraph, you said, "Free live accounts actually provide the setting you should use on this link." Does this mean that this is where to get the setting and not from GoDaddy?

Thanks again for the information. Now that you have the missing data, I would much appreciate anything you can supply that is more specific to the need.

I think you need to learn more from Godaddy. This is as close to instructions I can find on their web site. https://au.godaddy.com/help/set-up-my-workspace-email-domain-with-microsoft-365-19130

The delay here has occurred because of various lengthy and ultimately unhelpful conversations with GoDaddy "techs," reading carefully all the linked articles you set out (including the last one), attempting to see if the ISP can do anything (they can't/won't), and searching around for anything online that might have some magic bullet. I even spoke at great length with an experienced techy friend who also uses Thunderbird; he couldn't figure out what to do.

What I have is Office365 web mail that works just fine with my domain-based email accounts but only incoming email in Thunderbird. A gmail address works as it should in Thunderbird, however, and an old email address with my previous ISP's domain worked fine in Thunderbird, too.

The settings I had pulled up at the very first are what the ISP--Spectrum--said were their smtp settings, except for one, which is Authentication Method, which they said they do not have. A "tech" said that this is not a setting in their system but is encrypted. She suggested trying the "No Authentication" choice, which I had tried before (along with the rest of them), but I tried it again with no success. She said the problem was with the email client, not with Spectrum.

The GoDaddy "techs" went through the list of smtp settings, which were what I had pulled up at the very first (but subsequently had tried every possible combination of them before giving that up). They are: (1) Description--Office365 (Microsoft), (2) Server Name--smtp.office365.com, (3) Port--587, (4) Connection Security--STARTTLS, (5) Authentication Method--Normal Password, and (6) User Name--(my my domain-based email address).

The GoDaddy folks let me know that web mail was working just fine and that they were not responsible for third-party clients, who must be applied to for assistance.

I cannot be the only Thunderbird user who has encountered this. The situation is becoming insupportable. There just has to be some way through or around this. I am urgently hoping that I don't have to surrender and give up Thunderbird.

Thanks for persisting.