1. You can gzip strings before storing them in a DB
But many modern Databases/Filesystems have the built in ability to achieve the same effect - but faster and more transparently.
2. Verify an email’s host address is a valid DNS record.
True, you can. But what's the point? I see two possibilities - either you want to guarantee an address is legit or you don't. If you want a guarantee this doesn't get you anything (you'll still have to send a confirmation email). If you don't care, then what's the point (someone has to type joe@aol.com instead of joe@klasjfklashf.com?). And (as a more minor concern) at sufficient scale, it could cause performance problems. I remember some webserver used to reverse-lookup IPs so it could log hostnames in its access log, and it was brought to its knees by even minimal levels of traffic).