A few months ago I also moved a personal domain I was using for mail to another ISP. 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 <i>reuses</i> 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 <b>Outgoing Server (SMTP)</b> 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…