By default SharePoint 2007/2010 will not start workflow when an email coming to a document library or list library. But it can be enable by using STSADM commands.
stsadm -o getproperty -pn declarativeworkflowautostartonemailenabled
If this show below result
"Property Exist=”No”"
It says workflows don’t start automatically on lists that are E-mail enabled.
So you needs to change it to true.
stsadm -o setproperty -pn declarativeworkflowautostartonemailenabled -pv true
result
"Property Exist="Yes" Value="true""