Office 365: Maximum Number of Unverified Domains

Apparently there is a limit of how many unverified domains that you can have within an Office 365 subscription, and the magic number (as of 1 March 2016) is:

55

When you add more than 55 domains using PowerShell, you get the following error message:

Number of unverified domains exceeded.
Your account has too many unverified domains. Verify or delete one of your unverified domains, and then add the new domain.

The following article talks about the error, but does not mention the limit:

https://support.microsoft.com/en-us/kb/2279117

This article mentions more errors when adding domains into Office 365:

https://support.microsoft.com/en-us/kb/2762161