550 Request failed; Mailbox unavailable

All of a sudden, as of about the last week or so, I get this "Mailbox unavailable" message when I try to send an appointment entry.  I'm using Outlook 2007 and, so far as I can tell, when I send a normal email message, it goes through just fine.  But when I try to send an appointment entry using Outlook's Calendar function, I get this message:

      Your message did not reach some or all of the intended recipients.

      Subject:    Christmas lunch

      Sent: 12/2/2017 11:51 AM

      The following recipient(s) cannot be reached:

      [Name] on 12/2/2017 11:51 AM

      550 Request failed; Mailbox unavailable

Oddly enough, folks can successfully send a Calendar entry to me.  Any help you can give me would be greatly appreciated.