I'm not sure how it works in emails. I do all my work in SharePoint Web Part forms, yet I would think the principle remains the same.
In infopath have a connection to your data source (SharePoint list). This assumes your email is connected to the network.
My list has Title (single line of text) and Username fields (person/group)
In my test that I've just set up I have a field called current user and another field called Filtered Site.
On my field called Filterd Site, I have the following:
Secondary Connection (SP List) Highlight Title (the datafields one)
Filter Data = Display Name (Secondary Contection - SP List - datafields UserName) is equal to CurrentUser (Main Data Connection).
For this I have assumed a user has only 1 site available.