• the issue is with your SQL service not able to "lookup" the AD account... Here is the cmd that DBMail runs:

    EXEC sys.xp_logininfo

    @acctname = 'Domain\User'

    If this command kicks back an error... your dbmail is going to fail. There is an option in AD to hide a user to prevent someone from being able to list all the AD users. If this option is enabled for the user in question... DBMail will fail. MS knows about this issue and as far as I know... has no intention of fixing it.