Do you have a signature image or other attachment? It sends instantly without, fails with [on the desktop web client]. The google drive server they save our stuff on is giving a 503.
Hmm odd. I had a draft from this morning (I took a photo on my phone, attached, then went to desktop later to type the rest up). It would not send and kept popping up that error you mentioned while typing. As soon as I deleted the attachment, I could send it. New emails with nothing attached work fine too in all 3 accounts.
No images either!