What happens if the backup mx server is no longer in service?

If the backup mx server (backupmail.xyz.com) is no longer in service,  but the mx records stay the same as below:

10          mailserver.abc.com      104.x.x.x
20          backupmail.xyz.com      104.x.x.x

What happens if the primary mx goes down?  What will be the experience?  Will it have a bounced email?  What options do I have to prevent this from happening?
Norman Manzo NoraAsked:
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If none of the mail servers indicated by MX records can be reached, the delivery will be retried later. Some mail systems will report this with a "delivery delayed" message. After several attempts (could be a couple of days), the "delivery failed" messages goes back to the sender.

Outfits that sell mail hosting may also be willing to set things up so you can use them in an MX record -- they'll receive the mail if your other MX servers are down (in order of priority) and forward it on to you when your servers come back online.

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Norman Manzo NoraAuthor Commented:
Thanks jmcg.   I guess another mail hosting is the only option we have.  :-)
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