I assume that an SMTP relay server for a local TV station does not 
need to support more than 5000 users. I don't think Sparc/Solaris can 
compete performance or reliability-wise at that level [1]. The performance
and reliability bottlenecks are both going to be the disk, which you
can improve more cheaply on the x86 platform with fast SCSI + RAID.

I'd go with something like a PIII 550+, 256MB RAM, 10/100 NIC, Ultra160
SCSI card + RAID controller, then 2-4 10k+ 18GB SCSI drives depending on
your budget. 

The OS should be whatever OS you or whoever will support the machine 
will administer properly, and supports the minimum feature set (probably
MTA + packet filtering).

[1] Not a paid employee, spokesperson or shareholder of SUNW.

-- 
Michael