Increase Reliability of In-House Mail Servers
If you operate a mail server in your office — for example, if you have an Exchange server — ask your friendly IT technician if you have backup mail servers in place.
Commonly, a mail server is set up in an office, but if that server goes down (loses power, loses internet connectivity, etc.,) emails sent to that server will start bouncing back at the sender. The longer your server is down, the more likely the emails will bounce back.
This is rather unprofessional, but the easy answer is to configure backup mailservers. Usually, your Internet Service Provider or your web hosting company will allow you to use their mail servers at no additional cost. The backup mail servers will keep trying to forward emails through to your on-site mail server, and will buy you more time before emails are bounced back.