Actually, I don't think I'm missing the point. There are 5 other posters besides the orininal poster that have placed their problems here, some slightly different than what the OP posted.
I can create a new message, click to check before sending. I create multiple errors in a new message, click send, spell check comes up, then change some words manually in the message body, change some in the spell check box. Both methods work for me. When all errors are corrected, if you hit find next in spell check, then it sends automatically and the spell check box closes. Note: when it see's no more errors, then it sends after you correct the last error shown or when you hit find next and no errors are seen by spellcheck. It would be nice if you got a window saying no errors left with an option to send instead of being automatic.
Using spacebar and tab does not send the message. I have just tested this again with 3 messages. All worked as I have described. I can also bring up the check document now at anytime and close it and continue with the message.
I don't like the option of check before sending and don't use it because if it doesn't recognize the error, it will send when it thinks all are correct.
If what I've described isn't how it's working, then there is some corrupting somewhere.
If you choose not to check before sending and choose one of the other option, you can still select to check document now from the Edit Menu Spell Checking and not worry about it sending until you click send.
The OP posted his spell check window was not closing after message was sent. I believe Skillet was having problems not being able to get the other options such as "check as I type" to work.