I also fought this a long time.  I found the rest of the answer here:

https://www.sqlservercentral.com/forums/topic/t-sql-problem-using-run-as-user-in-job-step

So, you have to:

use msdb

CREATE USER [Domain\Account] FOR LOGIN [Domain\Account];

exec sp_addrolemember 'DatabaseMailUserRole', 'Domain\Account';

In the T-SQL job...

• This reply was modified 4 years ago by david.boerwinkle.