I experienced the same bug, twice now, pretty damaging. Messages appear to get sent, including the sound played with sending a message, but then they just get deleted and disappear completely without a trace and never arrive. Ouch.
FWIW, resetting the network preferences as suggested in this thread fixed it. For now at least, hope Apple fixes this critical bug soon, doesn't make me very confident
After spending 4 hours today trying everything to fix this problem, I finally fixed it. I went to other (not gmail button) and entered my email account as a POP instead of IMAP. This came up with all of my emails, starting with the oldest. I really just wanted to see if I could send an email with that and I could! Once I sent the email, I deleted that account and tried to set up the same gmail account but with IMAP instead (again, not using the gmail button). IT WORKED! Once I did that I tried what would happen if I used the gmail button (since it shows me sent mail, etc.) and that worked too!!! I could not be more excited right now. The link below will show you what you need to enter when you go through the other email button. GOOD LUCK!