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If you don't have access to your old computer, the only way you may recover deleted mail is through comcast.net website. Visit www.comcast.net, log on using your id and password, go to your webmail page. Check your inbox and the deleted mail or (trash) folder. If you're lucky your deleted mail could still be there. If you can't find in any of the folders it means your mail is gone. There are howver few cases where the mail servers can retrieve deleted mail for their customers in very special circumstance. If the deleted mail is important to you, try contacting comcast directly and see if they can help.
Hope this helps.