The longest email address can be 64 + 1 + 255 characters long (user name, @ sign, domain name) according to the IETF.

Memo fields can be useful when you're in a situation where users want to add multiple email addresses to one account. That way you don't have to add email1, email2, email3, etc.

The downside is that memo fields don't have indexes which means searching by email address can be slow.