Can you please simply paste the alert message here or better upload a screenshot of it?
Got it! Thank you for the effort.
Sorry, someone locked the question so I could not input anything just now.
Was it a Thunderbird error message?
From your Thunderbird, Go to Tools > Options > Advanced > Config Editor and make sure that mailnews.force_charset_override is set to false, then right-click on your Inbox and choose Edit > Folder Properties and make sure that the checkbox “Apply default to all messages in the folder…” is not checked.
The software should not be designed to show an error message that way. :(
You mean you even can't close the Thunderbird app?
Please advise your OSX version and Thunderbird version.
You may get the Thunderbird's version inform by checking its app (icon) in Application folder using Finder. Let me know the version so I can test it from my side.
Just moment, I am making a screenshot for you.
Okay. Thanks. Bear with me for a moment. I am going to install the same version here.
FYI - the latest version is 45.4.0 released on October 3, 2016. I am looking for your specific version...
Sorry, I could't find 38.5 for Mac, only the Windows version can be downloaded from a third-party site. It seems the official site only offers the latest version.
I am wondering why your Thunderbird is not up to date, as by default it should be automatically updated.
It is always recommended to keep up to date, but I am wondering this can't be directly fixed. Per the Wikipedia wording, "email addresses are currently limited to ASCII local parts", but anyway your issue looks like a bug, not support limitation (because I think all your email addresses are using standard ASCII characters.
FYI - https://support.mozilla.org/en-US/questions/1069657
Okay, I would suggest you upgrade your Thunderbird to the latest version, after well backing up the current emails.
Do you know how to back up email data?
Good to know it worked.
It is still recommended to upgrade your Thunderbird to the latest version, as a best practice.
Is there anything else that I could help with?
You are welcome.