I had pop account for years. Just created a new exchange account within Thunderbird. Both have the same email address. Trying to migrate all sent mail (thousands of emails) to my new sent folder so I dont lose them. When I move or copy to from old sent folder to new sent folder, it will only let me do a few at a time, and it dates them all to time and date that I moved them, not their origin dates. This process, although slow, worked great with my inbox, but not with sent. PLEASE HELP. Phone number is xxx.xxx.xxxx if needed, please email or call. Thanks for any help!

.edited phone# from public and search/spam bots view as this is a public forum.