demo here : https://regex101.com/r/hE9gB4/1
(?i)<.*?@(?P<domain>\w+\.\w+)(?=>)
its actually getting your domain name from the email id:
(?i) makes it match case insensitive and
?@ is nothing but @ which matches the character @ literally.
the ? in your ?@ is part of .*? which we call as a lazy operator, It will give you the text between the < and @
if you dont use the ? after the .* it will match everything after < to the end. ( we call this as the greedy operator)