Indexing

[Eric Savage]

We run an email account management system with mysql.  We use multiple
domains, so our main user table has a unique index on the user/domain
fields.  However, I do alot of work with just the user field.  Should I
create another non-unique index on just the user field, or would this
provide no/worse performance?  This is a varchar(21) field and alot of
substring searches are done.