Sendmail usernames

On a default setup system for 7.3, is there something special to do, to use
characters other than letters so that sendmail accepts them and longer than
8 characters?  A friend tried with a . (period) and with 10 characters in
the username and would get the *no such user exists) with them.

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Mike Chambers
Madisonville, KY

"Better to hurt a little now, than to hurt a lot later!!"