Who Me Too'd this topic

Whenever I try to send an email from MS Outlook using my Verizon account (and only from my Verizon account) I get the following message: 

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

      Subject:     FW: Hello from Oz (and other assorted topics)

      Sent:  6/3/2019 8:17 AM

The following recipient(s) cannot be reached:

      '{edited for privacy@}gmail.com' on 6/3/2019 8:17 AM

            550 Request failed; Mailbox unavailable

I have triple-checked my account settings; all server and folder settings are correct. Email works from all of my other accounts (another AOL account and a third-party account); verizon is the ONLY one that returns this issue. Can someone help, please?