A few months ago I also moved a personal domain I use for mail to another hosting provider. IONOS is one of the options I considered, but discarded precisely because it didn't seem to allow having multiple email addresses under a single mail account, so I very much doubt you'll be able to accomplish what you want with them. You may have to create different account settings (different username) for each email address you want to have there, and maybe even for both the incoming and the outgoing server.

Doing that for the incoming server settings may be tedious but should be straightforward. Getting things right for the outgoing server settings is going to be trickier, though, because of the confusing way in which Thunderbird handles this situation.

When you setup a new account, Thunderbird reuses existing outgoing server settings that may already exist as a result of having previously created another account for the same server (but different username / email address), without telling you those settings are being shared across multiple accounts nor giving you the option to create new separate outgoing server settings specific for this account during the account setup process.

Thunderbird lets you create and manage multiple outgoing server settings for the same server, though, just not from within the main account settings. You have to do that from a separate Outgoing Server (SMTP) settings pane you may open by selecting the corresponding option at the bottom of the list of accounts in Account Settings. The following article has a screenshot that illustrates what I'm talking about:

https://support.mozilla.org/kb/cannot-send-messages

You can add there as many settings for the same outgoing server as you need, with each entry having its own username, and then assign the right outgoing server settings to each account in the main account settings.

So make sure you get all of that right first, then we may look at other issues you also seem to have…

Thanks but it didn't help. I have checked using all user names and made sure that both the incoming and outgoing server settings are set to the new provider AND re-entered the passwords but still cannot send emails via Thunderbird though I can through IONOS webmail. Any other ideas you have would be appreciated. Thanks again

To ensure we're all on the same page, I suggest submitting screenshots for all four incoming server settings panes, all four SMTP server settings panes, and all for account panes (as they show which SMTP server is assigned), and complete text of error messages. That will help a lot.