given this look like part of a link, does it disappear when you type it or only after you click send. I am wondering is a helpful anti virus program is sanitizing your emails, or Thunderbird is trying to make them into links.

Dear Matt:

Either letter [x,y] disappears as I type. This started several days after I updated Thunderbird on my newer Mac. No problem with receiving items. I have no antivirus programs other than Apple's own protection & Antivira.

Sincerely/Mark

oops, should be /Avira/

If you are composing plain text messages, surrounding text with /, is a way to italicize the text. Does the same thing happen with HTML-format messages? I don't see the problem here on W10 with plain text or html mail.

I do not customarily send HTML-formatted messages. However, in an HTML test I just tried, the text did not lose Uppercase Y & X, but still dropped Lower Case.

Others have reported weird letters that disappear in a Mac - not necessarilly when using Thunderbird, so it's worth checking to see if you have some setting like the people at these links:

• https://discussions.apple.com/thread/8588270
• https://support.google.com/chrome/thread/2892761/the-letter-y-is-missing-on-some-pages-i-can-look-up-the-pages-in-another-browser-and-they-display-fine?hl=en

Or something going on with trackpad or battery:

• https://discussions.apple.com/thread/3036873

The missing letter problem only occurs in outgoing eMail via Thunderbird, & only affects lower case x & y. I have another, inferior eMail server in which this error does not occur. My machine is an iMac & there are no trackpad or battery issues.

Try clearing the font cache:

https://osxdaily.com/2015/01/08/clear-font-caches-databases-mac-os-x/