I am approaching the limit of allowed comcast.net email(10G) and do not have enough time to delete emails as I am dealing with a number of other personal issues now. If I switch to another email client such as Mozilla's Thunderbird, will the new emails still be sitting on comcast's severs or are they on Mozilla's? Also would I still be able to access the old emails on Comcast's servers as I have a number of important email files there? Finally is there a way to have my emails reside on my PC instead of some ISP's servers? Any information would be gladly appreciated.
Assuming you use IMAP in Thunderbird, everything is synced. Comcast accounts are IMAP compliant by default. You can configure it using POP3, but it's not recommended if you access your email from more than once device. That said, most desktop email clients like Thunderbird have the ability to create folders that are local only to that copy of Thunderbird on that particular computer. So those would not be affected by IMAP. What this means is yes--------you can store emails in T-Bird and not have them on the Comcast servers. In Thunderbird it's just called "Local Folders" and is at the botttom of the folders pane.
To create a local folder, just right-click on "Local Folders" and you will see that option. You could then move or copy emails from any other folder to that local one. If you only copy them you would still need to delete the originals from the server to free up space. So in your case you would want to move them, which will remove them from the server. Also, you can drag and drop an entire folder you may have created in the account to that local folder. That will copy it, and you will still need to go back and delete the original one to free up space.
Also, as to your total limit in the Comcast account, don't forget to look at your Sent and Drafts folders. There can be a huge amount of email in there.