I noticed messages were not being sent from the Gmail app. I checked to make sure auto sync is enabled and it is. Any ideas? Beside the message it just says sending. I have other email accounts used by the Gmail app (e.g. yahoo) and they work fine. Also I can send and receive find from Outlook on my computer. I don't know what else information to give so feel free to ask for more. I'm using Xperia Z2 with Android 4.4
Have you tried re-adding your Gmail account? It is also possible that if you are on a work or school wireless network that they have blocked insecure outgoing connections, i.e. SMTP Port 25 or 587. You could check if this is the case by seeing if your outgoing mail settings are set to Port 465 with SSL.
I just had this problem and the problem was that I was over my email quota. Specifically, I was using the GMail app to connect to a Microsoft Exchange Server and sending from my desktop (via thunderbird) worked, but sending via my phone didn't.
I logged into the outlook web-access portal and it gave me an error message saying I was over-quota. Fixed the quota issue now I can send email.