Updating ssl certificate server 2016
This is because all those domains deploy SPF and DKIM anti-forgery measures, and faking your from address is forgery.
If you set or higher, you will see what the remote SMTP server says.
Very often this will tell you exactly what is wrong - things like "Incorrect password", or sometimes a URL of a page to help you diagnose the problem. Google does this a lot - see below for info about their "Allow less secure apps" setting.
Some service providers (including Digital Ocean) provide IPv6 connectivity for servers but block outbound SMTP over IPv6 while allowing it on IPv4.The solution is extremely poorly documented by Go Daddy: you must send through their servers, and also disable all security features, username, and password (great, huh?!), giving you this config for PHPMailer: address belonging to any aol, gmail, yahoo, hotmail, live, aim, or msn domain (see their docs).Popular US hosting provider Go Daddy imposes very strict (to the point of becoming almost useless) constraints on sending an email.They block outbound SMTP to ports 25, 465 and 587 to all servers except their own.