One exchnage email on mutiple outlooks?

Hi all

I have exchnage 2007 env.

My question is about using one exchnage email account (e.g. hr@mydomain.com) on mutiple machines/Outlooks.

Now I have 3 users using the (hr@mydomain.com) in addition to thier exchnage account in outlook,

the users keep complaining about emails stuck in outbox for the hr@mydomain.com account.

Is there any consideration/setting/limitation shoud be considered when using the same account in multiple outlooks?

Thanks

Ayman