https://datatracker.ietf.org/doc/html/rfc5322 

On an Email message retrieved by Thunderbird from my Gmail account, the sender-name shown in my Inbox is different from the sender-name shown when I look at the message directly in my Gmail account. When I reply to the message via Thunderbird, it uses the correct sender-name. I assume that there must be at least two fields in the Email message that would identifying the sender-name of the message. I tried to find out more from the RFC5322 specs here: https://datatracker.ietf.org/doc/html/rfc5322 My guess is that Thunderbird is getting whatever it displays as the sender-name in my inbox from a different field from what it uses as the "To" name when I reply to a message. For most messages, these two fields are probably the same, so there's no noticeable difference. But attached screenshots illustrate the difference: [1] Thunderbird Inbox wrongly shows Sender as "Dan Sullivan" (deceased husband of sender, who may have been the account holder when the Earthlink Email address was setup many years ago) [2] Gmail Inbox show Sender as "Margaret Sullivan" (writer of the message) [3] Thunderbird reply correctly shows the recipient as "Margaret Sullivan" This isn't a huge problem. (When I pointed out the oddity in my reply to my friend, I told her that "(Personally, I enjoyed the reminder of Dan's charming smile.)" But it probably should be added to the list of things to tidy up whenever anybody's modifying the relevant component of the Thunderbird program. Of course, my friend can probably prevent it from happening by changing some field in whatever software she uses for writing/receiving Emails, but that might be more trouble than it's worth for a non-geek. Happy New Year! Thanks to Thunderbird for providing the Email client software I've been happily using for many years, and hope to continue doing so for many years to come.