Hello,

Here's the context, quickly:
We wanted to receive SMS when a ticket was assigned to a specific group, with a specific priority. The sending of the SMS is done by our SMTP gateway, not in Redmine, but we needed the SMTP headers anyway. Our Ruby guys being busy, we didn't take the time to develop a proper Redmine module to send SMS, sorry about that ...

I manually edited our Redmine 3.0.1 to add

'Issue-Priority' => issue.priority.name,

in mailer.rb so X-Redmine-Issue-Priority was added fine.
I believe this wouldn't hurt to be added by default in Redmine (I apologize for not providing the patch, I have no idea how to do that ...).

However, there was already a X-Redmine-Issue-Assignee, as

 redmine_headers 'Issue-Assignee' => issue.assigned_to.login if issue.assigned_to

but as there is no "login" for groups, the header was empty in my case.
I changed it for

redmine_headers 'Issue-Assignee' => issue.assigned_to.name if issue.assigned_to

because, in my case, that suits me well, but I understand that as there might be several groups or users with the same name, this is not a reliable solution.

If anyone has an idea about that, please feel free to express yourself =)