We have a total of about 7 users, all but 1 of those users can send email.

We are using mail transfer agent: SMTP, not using the outlook plug-in.
Everyone's settings are the same, everyone has administrator's access. all users are Active.

I've gone over everyone's setting over & over but can't find any difference between the ones who can send and the one that cannot send. I'm completely stumped.

Is there something I'm missing?