Help is not helpful! Emails won't send to my customers

Help is not helpful! Emails won't send to my customers

I have been having issues for several days - I able to send emails to my customers without having a return demon or error report - the problem is they're not receiving them! 

Having gone through the help suggestions online - the information it gives you is useless. 
It told me to amend some details in the email forwarding imap/pop tab in settings but when i click on it you can't actually change anything! 

WHY???????????????????????????????????????????????????