Error : 550 5.7.54 SMTP; Unable to relay recipient in non-accepted.

 If you are running Exchange and have a receive connector and when sending mails externally you get this when using the connector:

I have a connector called "OutsideBear" on the server "MailBear" but it cannot mail anything outside bears.cloud, we get this:

Error : 550 5.7.54 SMTP; Unable to relay recipient in non-accepted.

You need to run this to allow relay to external recipients via a connector:

Set-ReceiveConnector MailBear\OutsideBear -PermissionGroups AnonymousUsers

Get-ReceiveConnector  MailBear\OutsideBear | Add-ADPermission -User "NT AUTHORITY\ANONYMOUS LOGON" -ExtendedRights "Ms-Exch-SMTP-Accept-Any-Recipient"

Then you are good to go.