I was about to post a sad story about my recent Error 500 situation
But luckily you have been saved from that now 
I was tring to figure out the intricacies of configuring email.yaml
Next thing my site was broken - Error 500 - great sadness
I tried many many things, even going so far as reading the excellent Error 500 article on learn.getgrav, but all to no avail
Anyway a new morning - on clearing the grav cache (which I should have done first) a very helpful warning message comes up, directing me a line in to email.yaml
I had entered from:blah@asd.com instead of from: blah@asd.com
Added that space and we are all go again - just a space!
Just thought my recent woes might help someone in the future!