Sending email thru Outlook Express

System Setup:
Windows 98 2nd Edition
ADSL modem from PacBell using PPOE (Dynamic)
AMD Duron 750MHz
128MB SRAM, PC-133

I am using Outlook Express 5 and I can receive email, but I cannot send any email.
Whenever I sent email I rec’d such message:

Error #0x800CCC79 ; Server Error 550. Email file was inhibited by Adminstrator, Port 25 Secure (SSL).

I am running peer to peer networking using Windows 98 connecting 5 workstations. I get the same error message even when I use only one computer standalone connected directly to ADSL modem ( No router or hub ).