Please vote/promote this idea to add authentication to the email relay function.  Currently, you can only setup relay openly so no reputable relay service is available.  Just think, you could control your reputation and prevent blacklist issues.

https://success.salesforce.com/ideaview?id=087300000006wu7AAA

@Solo Admins @* Salesforce Administrators * @IdeaExchange Moderator @Salesforce Flow Automation @* Outlook/Teams, Gmail, and Inbox * @Commons Project: DLRS