MX records in DNS are used to route emails to correct mail servers and properly send to correct destination server. If you are using Google apps account for hosting email accounts on Google servers for your domains, then you need to add following MX records in your domains DNS configuration.
Google Apps MX Records:
@ IN MX 1 ASPMX.L.GOOGLE.COM @ IN MX 5 ALT1.ASPMX.L.GOOGLE.COM @ IN MX 5 ALT2.ASPMX.L.GOOGLE.COM @ IN MX 10 ALT3.ASPMX.L.GOOGLE.COM @ IN MX 10 ALT4.ASPMX.L.GOOGLE.COM
