Here are instructions on how to configure Thunderbird for the 2021 change to new servers.

From inside of Thunderbird, click the three bars on the top right, then click the top option "New". Then click the options that says "Existing Email Account..."

In the popup which should look like the following, fill in your complete email address and password that Msen provided, then click "Configure manually".

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In the second popup, change the inbound and outbound server to mail.b.hostedemail.com, then click Re-test.

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In the third popup, look for the green checkbox in the middle of the page.

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If this works, you are all set. If not, it in very likely that you entered your user-id or password incorrectly. Please insure that your userid includes the properly spelled server, that your password was entered via cut and paste and does not include spaces before or after the actual password.