I don't see that here (see picture). Which dictionary, and does it happen when you type directly, i.e. not pasted from outside source? Using an external spellchecker or spelling add-on that isn't a dictionary add-on?

Also, I don't see To boxes checked.

The next time it happens I'll take screen shots and post them here. I am attaching the Spell check settings for now.

Okay 'sfhowes', I tried sending emails with contractions and my spell check ignored them, so I am thinking it must be when I paste some copied text from the internet. (If it happens again I will grab screenshots).

However, Thunderbird is still trying to spell check the To: field. I am hereby attaching two examples. Please note, I have blocked their actual email addresses in the screenshot to protect their privacy. But you can see that Thunderbird is constantly trying to spell check the To: field. I have never had an email spell check do that before using Thunderbird. With how email addresses and names are not likely in any dictionary, Thunderbird should be programmed to not look there.

My dictionary (see picture) doesn't check the To box, even when I enter the same name as in your example. The entries show neither the flagging of misspelled words nor are they flagged when spelling is checked manually. Try with the US dictionary disabled.

Is there an external grammar/spellchecker, perhaps part of an office suite, or a TB add-on?

I have now deleted the US dictionary and have no other add-ons. I do have MS Office 2013 and use Word occasionally, but not at the same time as TB. My MS Outlook was causing other problems and so I stopped using it and switched to TB.

I will see if having only one dictionary makes a difference.

Thanks.