I'm afraid I don't remember all of the steps I took. There were many failed attempts to get this working.

The MSA (Managed Service Account) was created on the Domain Controller.

In SSMS:

use master;

GO

CREATE LOGIN [MyDomain\MyMSA$] FROM WINDOWS;

GO

USE msdb;

GO

CREATE USER [MyDomain\MyMSA$]FROM LOGIN [MyDomain\MyMSA$];

GO

ALTER ROLE DatabaseMailUserRoleADD MEMBER [MyDomain\MyMSA$];

GO

Prior to doing that, I got an error about the user accessing objects in msdb.