I also have had this problem

Was with POP3

I unloaded Norton’s Virus Scan Service and email immediately was able to be
sent.

I am using Symantec Antivirus Corporate Edition 2005 (v10)

I loaded the service again after the email was sent.

Oddly even though I am not using Microsoft Exchange by disabling Microsoft
Exchange Auto Protection in Symantec the problem went away and outgoing
emails are immediate.