email sent with php mail() function, it arrives around 12 hours later?

What is the first thing to do when email which are sent from php mail() function arrive 12 hours later to mailbox. So yesterday I sent email and after 12 hours or less it arriaves in my mailbox? Where to check first? I have also WHM access. I think problem was when we updated to 7.1 php version from php 5.6. Sending from php mail() function works, but sends the email later on like 12 hours later