It's not available with macros, but a little regular expression would do just fine. Here's an example that does what you want:

%SetPattRegExp=_(.*)@(.*)_%-
%RegExpBlindMatch=_%ToAddr_%-
%SubPatt=_2_

I copied this from one of my own templates, where it's used to extract the domain. I'm using an underscore (_) in stead of single or double quotes, because it was originally part of a more complex structure where the quotes were used on another level, but that doesn't matter in how you use it.
In case you'd need the user part, you'd need %SubPatt=_1_