What to do when MX server is not receiving email from webmail like google?

A well configured Mx does not present any issue until and unless there is some external issue.
Number of issue that can cause Mx to not receive email is if the domain expires and google smtp cannot find domain or find invalid domain.
Just a reminder to myself , if a client calls and tell you they are not receiving email instead of rushing to check mx server log just check if domain is valid and is pointing to your mail server or at least nameserver.