Mail sending Python - What it's wrong?

[gontajones]

Check your line 16:

msg['To'] = send_to_email

I'm afraid that send_to_email is a list here and msg['To'] is expecting a string.
You can test it passing just a dummy email ([email protected]).
If this is the error and you still want to mail every email on this list (send_to_email) you'll have to iterate through it and send one at time.