If you’re using Office 365 with Exchange and have been since before 2013, and occasionally discover you’re missing or experiencing delayed email, it may be due to your domain’s MX records. As of 1 January 2013, generic MX records such as mail.global.frontbridge.com were deprecated; using them beyond 15 July 2014 can cause service disruption.
To check existing DNS including the MX records for your domain, on a system that has dig installed:
# dig +nocmd yourdomain.com any +multiline +noall +answer
The Office 365 notice can be found here, which will link to the appropriate articles for updating your MX records.