alternate name for mail servers?

Hi, I have an existing Thunderbird email client loaded with mail and I want to upload it all to my new account that I've set up on my new inmotion server space. I haven't pointed my domain to inmotion yet, so I need another way to identify the mail server within the email client (can't be mail.dadmun.com). Do you know if there's an ip or other server name that I can use in place of mail.dadmun.com, to use in my mail client to upload my mail to the new server? I am using a the Thunderbird add-on Mailredirect, which will re-write the message headers to the originals and redirect them from the client to the new server. This has worked in test. I am hopeful that I can get this done before I point my domain from the old servers to Inmotion.

Thanks for any advise you might offer.

John