Can't get rid of an email that would not get sent and only get some of my gmail

I do not get all my Gmail. I get some but not all; it is random. ALSO, perhaps related, I tried to send an email with an attachment to a large number of people. I got the message "Cannot send message using the server Gmail." BUT I cannot get rid of this email. It always reappears, unsent, with the error message. I cannot delete it and try again. HELP