Database mail user not sending emails

Hi all

I have a user (User1) which is employed to send an email to an internal address when an event is triggered.  It's DB permissions are as follows:

• StorageDB - dbOwner
• msdb DB - DatabaseMailUserRole

When I instigate a trigger, this user should have access sp_send_dbmail to send an emil to a user.  This however is not the case.

In attempting to find a solution, I have given User1 sysadmin server role rights on our dev server and upon doing this, this now allows the user to send an email.

So there is something within the sysadmin server role which is allowing the user to send emails.

What can I do to allow the user to send emails WITHOUT sysadmin access?

Thanks in advance.