Mail 1.3.9 - Can't Reply


OS 10.3.8 - G5 iMac - Mail 1.3.9 - LinkSys Router - DSL Internet Connection (Sympatico) POP account

No problem receiveing & sending emails.

But attempting to reply to or forward a message results in a spinning cogwheel for 4-5 minutes - & then a Dialogue Box indicating that Mail could not connect to the server - message saved in out box.

Immediately able to send a new message but not the replied message.

Have repaired permissions, checked the disk with Disk Utility & Disk Warrior. Then did an 'Archive & Install' clean install of the System. Then upgraded to 10.3.9

Spent more than 2 hours with Apple Support- no luck!

Problem remains!

Able to utilize Web-based server offered by Sympatico & no problems replying to emails.

G5 Power Mac on same network & with same OS / Mail 1.3.9 works without a flaw.

Any suggestions gratefully received!

Many thanks!

...John :(
When launching, hold down shift-key.
Done already that, or would it do any difference?
Had not tried that.
But, no luck.
As before, 3 test messages were successfully sent or forwarded but the 4th hung up..cog wheel still spinning after 4 minutes!