To diagnose problems with Thunderbird, try the following. Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

Still doesn't work - but thanks for trying to help me.

did you have to click continue in safe mode?

yes I did - nothing changed

I've been using it for years and I don't keep an address book. However I never had that issue. I also have a fresh copy of TB in another computer with the latest version and it still works

Send button is disabled I type in john Send button still disabled john@ Send button still disabled john@a Send button enables at that point. Are you writing a correct e-mail address?

Yes, I'm hardly a novice. I've been using Thunderbird for quite a few years. It was just recently that his annoying thing started happening - after the last update. This happens even when just trying to reply to a message received - when I don't type in the address, but it self-populates from the senders reply-to address - and only if they're not in my address book.

Sorry for another silly question then.

The autocompleted address is correct? Can you copy/paste the whole thing in Notepad and check if it is right? no weird spaces etc?

It might be the other party who has a reply-to address set to "me@ email.com" and there are spaces after the @

I appreciate you trying to help. We're talking about over 6 weeks of consistent behavior here. If not in my address book, it won't let me send. not one or two weird bad address issues, unfortunately. I think I would have figured that out pretty quickly. The way it started happening after the upgrade, and so consistently made me wonder if there had been something put in the code that someone thought would be a security improvement. I looked all over in preferences and settings to try to find something that might be causing this, but couldn't. Then after a week or two, I downloaded the new Beta - out of desperation to see if maybe that might work correctly. So I've been putting up with this problem since mid-late May....

I can't help you. Maybe it would be good to try resetting all the config. I don't know if you have another computer or VM to try it. I use it daily and haven't had one issue and I use no address book. Something failed after an update maybe?

Can you revert to an older version? (before 52?)

Maybe that helps.

I must ask, what mail provider? Do they have setting on their web site to restrict mail to only some folk like those in the address book? Do you have an anti virus product? What is it?

We are all unable to replicate your problem, but we also do not have access to a copy of OSX. However I really doubt that the problem is Thunderbird or any of it's settings.

Click this email address and send an email to it. I doubt it will be in your address book. (it is mine that I use for testing) devilsgatedrive@gmail.com

What exactly happens when you click the link.