MD5 (Message-Digest algorithm 5) is considered the industry standard for encrypting email addresses. MD5 is an alpha-numeric hash that cannot be reverse engineered to show plain text email addresses, therefore making it difficult for an unsavory Affiliates / ESPs / Mailers or others to gain access to your list and use it for unauthorized purposes.
In regards to OPTIZMO
Some Advertisers want the added security that only MD5 can provide. This means the Mailers cannot see your email list (as each individual email is encrypted) making your list well protected.