This is a simple plugin that does the job well. Never had any problems with it at all..
Only improvements I could think of would be: - add a list of the "bad" email addresses to the alert in some easy to read format. Confirm-addresses 1.2.3 is for me a worse plugin, but has a slightly more "pretty" interface. - cope better with subdomains - when sending from firstname.lastname@example.org emails in both foo.com and bar.foo.com are both valid (for me at least), so having an option to allow this would be good - or perhaps just have two separate warnings (one for totally mismatched domains, one for those that just don't match the entire string after @). Currently the entire subdomain/domain has to match.
This review is for a previous version of the add-on (0.6).