I also thought that a shortcut where you have to press three keys at once was too difficult!
My solution was also in AutoHotKey. First you set the title match mode to RegEx because then you can match the titles through a regular expression. Put the following near the top of your script:
SetTitleMatchMode RegEx
And here is the shortcut which should only work in Outlook 2010 (note that I am using Control + Shift + 9):
#IfWinActive Inbox.*Microsoft Outlook
a:: SendInput ^+9
#IfWinActive
For Outlook 2013, the regex needs to be modified slightly:
#IfWinActive Inbox.*Outlook
a:: SendInput ^+9
#IfWinActive
