mac mail can receive but can't send messages

You are a star, I tried and tried everybody else's suggestions to no avail. Your answer worked immediately, Hallelujah!! Many many thanks. B.

Found a faster, easier, less frustrating fix. Click the lightening bolt button to "take accounts offline" then the envelope/"get mail" button. This should re-establish a server and fix the outgoing SMTP error.

Thank you so much, spent countless hours trying to fix this and deleting all the outgoing servers and re-entering the same information fix it!

Here's what seemed to work for me. You need to enter your password for both the outgoing and incoming servers. The incoming one is easy. For the outgoing, I brought up "edit outgoing server list", the hit "advanced". You'll get the option to enter a password.

This worked for me - Thankyou

In my case it was the fact that I had 2-step verification turned on in gmail (code sent to phone, was turned on) so it would not login. I turned this off in gmail and then my incoming mail streamed in.

For me though I will not use the Mail app on my Mac, I am more Google centric. I just needed to do this to be able to click on a link in an email a friend sent me to be able to view his iCloud photos, I could not click on the link in gmail I HAD to do it in Mail app (per the error message)... Now I did that and I will abandon Mail and turn back on my 2-step verification in gmail.