The scanning is internal and private. No external agencies or third parties are involved.

The feature can be turned off at: Tools|Options|Composition|General

Of course Thunderbird "scans" your messages. It can spell-check. It may convert your text to mime-encoded, or base-64. When displaying text. it "scans" it to find decorations.

Your anti-virus, and other people's, scan your messages for unwanted content.

Nothing should ever "look" at my email. Not spell check, not antivirus or anything else.

This kind of 'feature' is extraneous, unecessary. If this software is allowed to look then another application or addon/plugin will be allowed to look eventually too, so on and so forth until eventually we will have no control at all... just as it is with Gmail, outlook, Yahoo, etc... right now, already. I don't use web based mail because I'm trying to maintain some semblance of autonomy, anonymity and privacy.

Thunderbird is one of the last decent email programs available for internal use. I'd hate to see it turn to rubbish under the idea that these 'features' are wanted or desired.

That it is internal now means little. At the very instant that someone at Mozilla begins to think that they can add a feature to Thunderbird that does use an external DB or other outside (cloud based) application to scan or otherwise effect email in a way they think is a positive or a help, they'll implement it in the name of 'improving' the service, leaving the user open to God knows what kinds of security and privacy nightmares. I'd be willing to bet that these kinds of ideas have already been discussed at Mozilla, and furthermore I'd be surprised if these kinds of things are not already slated for future releases.

Just as it is with Gmail looking up tracking numbers, flight schedules, birthdays, etc... I don't want any of that. That Thunderbird even hints at the idea that it might look at things like this is a giant turnoff.

Give an inch and a mile will be taken.

I disabled the 'feature' through about:config and removed the list of keywords as well, but I suspect more action will be necessary on my end to feel comfortable that I can use Thunderbird and be comfortable with it.

Spell check is dumbing people down by the droves. No one bothers to learn how to spell anything anymore because they rely on spell check. I'd rather misspell a word get called out for it and then learn the correct spelling than to remain ignorant.

Folks don't seem to be able to take in the larger, long term picture.

I appreciate your time and response. It was a help to know that this is actually in the options menu.

I've heard it said that email is about as secure as a postcard written in pencil and sent through the mail. Anyone en-route can read it and possibly even edit it without you being aware.

The vast majority of email travels in plain text, or in a form such as base-64 that is trivial to decipher. Even connections with TLS/SSL encryption generally apply this encryption only to login data, mainly as a precaution against identity theft. If you want end-to-end encryption, use s/mime or gpg or similar. Arguably, encrypted email might be more interesting to agencies such as NSA and GCHQ and their Five Eyes chums than plain text messages. And we learn from Snowden that these spooks have back-door access to SSL and so on anyway.

Given your stance, I'm surprised that you use email at all.

I use a spelling checker because I am a bad typist, not a bad speller.

I'll warn you now that Thunderbird "scans" your addressees and tells you which ones it can't validate against the address book, by printing them in red.