Problems with e-mails

Hello guys! I migrated my websites from Plesk to cPanel, since then, my sites' Contact Form are not delivering emails that are sent to 'external servers' like Gmail, Hotmail etc... I just receive the messages when it's set to send to the domain email created inside cPanel, like support@domain.com.br. But the thing is just ONE of my sites is delivering messages to any email address. I use the same Contact Form in every website. Every new domain user I create inside cPanel comes with this 'pattern', it doesn't deliver emails to 'external servers'. I don't know much about hosting, so I don't know if it's something I can change or an internal problem with my hosting provider. Could someone give me an advice?