Can't send email for 5 days from MacBookPro
I've been sending email through MacMail with my current settings for years. Suddenly, a few days ago my email is not sending. At first, I got a message that my server was invalid (I haven't made any changes.) Now I'm not even getting that message. Still, none of my email is going out. At the bottom of my email screen where it shows "connecting, downloading, etc.) I'm getting the message "Sending Message." But it never changes, like it's hung up on that step.
I'm receiving email, but don't know if it's all of my email or not.
Using mid-2012 MacBook Pro, OSX 10.11.6, Safari 11.1.2
Can anyone suggest what could have happened or how to fix this. I'm not ready to upgrade to Sierra, yet
MacBook Pro, OS X El Capitan (10.11.6), mid2012 MacBook Pro, Safari 11.1.2