The SMTP in apple mail is not sending and the email is stuck in outbox.

I have tried all SMTP settings but they won't work. 

I have tried contacting the support (Ref:  ##11240139##)  but after a couple of weeks still no solution could be provided.

It was suggested that I visit this website as it might be an apple problem:  http://osxdaily.com/2014/10/28/fix-mail-smtp-sending-errors-os-x-mac/

But this didn't help since I couldn't find the zoho accounts in the accounts.plist file but only my other hotmail account which works fine. I have also send this file to zoho support, but no answer.

I have also updated to the latest OS but no change.

My colleagues using Windows and outlook have no problems sending.

Thank you.