what antivirus do you use? Beyond windows defender, they tend to be so aggressive these days they actually prevent the setup process. I find restarting the operating system in safe mode with networking and a wired connection almost always works without issue. But I have had to resort to physically removing Norton products to actually get rid of the problems that product caused.

Just Windows Defender. I haven't used any other antivirus products for more than a decade.

Here's a screenshot. It sits here literally forever. I cannot move on from here, only Cancel, which cancels the account setup entirely.

On their support page, they state that email accounts don't become active until the DNS configuration is completed. Could that be the problem?

I think the question is, what is your custom domain? You tried to obscure the email address in the image you posted, but I simply do not see where there might be a period in that address at all. Likewise I have never seen an address in the format user@host@emaildomain

Please post the full domain you are trying to get mail for and any errors or messages verbatim

To sfhowes: No, even long after the DNS configuration was complete (I am able to send and receive emails from their webmail) I am still unable to set up Thunderbird with this email address.

To Matt: There is a period in the address. And the error *is* the email address @ the email domain, hence why I listed it that way ie me@mydomain.com@imap.emailserver.com.

The custom domain itself is irrelevant as it only counts for the login information.

I was able to get it working by creating a new profile and deleting the old one.