No address
Next line is blank

someone@somewhere.com
Line above matches at start, line below is indented:
        who.name@where.com

What about multiple matches?
first@first.com and second@second.com

What about dangling @ signs?
@nomatch.com
nomatch@ space after '@'

Let's try quotes: "a@b.com", <c@d.com>
And quotes in the middle: a'b@c.d

Longer domains with funny characters: p@q1.r-s.t-u
Short domains: x@y