I am running Thunderbird on Linux, have used it for perhaps 15 years and never had a problem sending or receiving messages. I have one client who has windows computers who says that the messages I send to him display as plain text. He says that messages from anyone else are fine and insists that I am sending plain text or that the email program is automatically doing this.
I tested by signing into several Windows computer (W10 and W11) set up Outlook and the default Mail program , then sent a batch of minimally to highly formatted emails from my Linux Tbird to the accounts and see no problems. They are always perfectly formatted.
I don't see a problem on my end? Have you heard of this? Could he have accidentally messed up his settings to cause the issue? Thx