HELP! sp_send_dbmail error

I'm having trouble sending an email through DBMail. I'm logged in a a SQL Login user named "ServiceUser". I have added this user to the msdb database and as a rolemember to "DatabaseMailUserRole"

USE [msdb]

GO

CREATEUSER [ServiceUser] FORLOGIN [ServiceUser] WITHDEFAULT_SCHEMA=[guest]

GO

USE [msdb]

GO

EXEC sp_addrolemember N'DatabaseMailUserRole', N'ServiceUser'

GO

When I try to send the email:

exec msdb.dbo.sp_send_dbmail

@.profile_name=@.DatabaseMailProfileToUse,

@.recipients=@.EmailAddress,

@.subject=@.Subject,

@.body=@.MessageBody,

@.mailitem_id=@.MYmailitem_id OUTPUT

I get the following error:

Msg 15404, Level 16, State 10, Procedure xp_logininfo, Line 62

Could not obtain information about Windows NT group/user 'ServiceUser', error code 0xffff0002.

I executed the following on msdb and it didn't help:

execsp_changedbowner'sa'

xp_logininfo also returns the error:

EXECxp_logininfo'ServiceUser'

What is wrong? HELP!

My database mail profile was not public! It needs to be public or set to private for the user that is going to access it.