MAIL FIX WHEN DEALING PARTICULARLY WITH GOOGLE VS. MAIL SEND/ RECEIVE ISSUES
*This fix worked specifically when dealing with Google Email Accounts (Gmail or own domain), but principle may apply with other email clients (Yahoo etc). For some reason POP emails are having major issues, but IMAPs can be fixed.
Ok, so here is what worked for me. If send/receiving is causing you issues follow these steps:
1. First, any important emails from your inbox you want to keep, create a folder in the Mail left hand pane (in right column under 'On My Mac' > right click to create folder), and move any emails you don't want to be lost into this folder on 'On my Mac'. Moving your emails there should keep them safe, as deleting email accounts will delete your inbox data, unless they are stored on the Mac (in theory).
2. Next, goto Mac settings > Internet Accounts and delete all your email accounts (Note - this will delete all inbox data - hence why step 1 important).
3. Now in Mac Mail, goto Preferences > Accounts. Click the plus to add new account. Re-add all your email accounts.
This is one simple fix.
Even before doing this you can try:
1. Goto Mail > Preferences > Advanced. In advanced make sure the box 'automatically detect and maintain account settings' is ticked.
This can be a quick fix for those using POP, but it would seem there is a code issue between Google servers / POP / Mail / El Caption, so may have to wait for the update before this is resolved. It would seem El Caption as screwed up backend settings of account data so deleting accounts and re-installing so to speak is one solution.
I was using POP (still trapped in the dark ages lol), but thanks to our jolly El Caption, who steers his ship about as good as Jack Sparrow, I have leaped into using IMAP.
Note - Might be wise to go into your email client settings (gmail or whatever) before doing this, and wherever the POP/IMAP settings are make sure your 'folder size limits' are set to a low number, like 1000 or 5000, as re-adding accounts often can cause the server, Gmail for example, to download 50,000 old emails. And if you have 5 business email accounts like me, have fun sorting them lol!