Amazingly, for me, the solution was:
1 - Deactivate Postman. It turns out the problem I was trying to fix by installing Postman could not be solved in that way.
2 - ADD MX Entries in CPanel, adding (for GoDaddy, your host will be different):
0 smtp.secureserver.net
10 mailstore1.secureserver.net
IMPORTANT - these are not changes to be made to your domain's DNS / Zone File - add to the "MX Entry" section of your hosting account's CPanel.
My PHP form (Jetpack in this case) is now working perfectly.
I know this is an old thread, but figuring my solution may apply to many as GoDaddy hosting is so common. Good luck!
