Thanks for moving and sorry for the delay!
Marketo doesn't offer this kind of parsing natively, but a scripting layer like FlowBoost* can do it easily in JavaScript:
var domain = ( {{Lead.Email Address}}.match(/@(.+)/) || ['N/A'] ).pop();
Just a little snippet that returns the domain or the string "N/A" if not found. You could leave it blank instead... your call. You can also read about the extended FBEmail module, which does more than just parsing, over on the blog: http://blog.teknkl.com/flowboost-fbemail-for-domains/
* Yes, I'm the tech lead of FlowBoost and related SaaS stuff.