RedHat 7.3 ISP

I'm a small and upcoming ISP... I'm using RedHat 7.3, 256 RAM, 30GB SCSI drive, 
Compaq Proliant 400 server PII Server for my dial-up customers, I have around 250 
dial-up customers, runing a 192k line. Runing NAI McAfee AntiVirus (uvscan) as my 
antivirus. My problem is when I'm using a mail client like outlook/express my mail 
takes for ever to go, this is not only with the dail-ups but from network to. From my 
network it takes a full minute and 10sec to connect and then it starts sending and 
this is making my xustomers complain. I'm runing the Sendmail, the sendmail that 
comes wih RedHat 7.3