[imp] Can't send email

[Adam Dershowitz Ph.D., P.E.]

I just installed horde and imp on a CentOS system.  I am using imp-h3  
4.3  and horde 3.1.7.
Most things are working fine.  I can read imap email and such as  
expected.  But when I try to send email it doesn't work.
I have horde set to use localhost for the mailer.  I also have imp set  
to use localhost for the smtp server.
When I click to send a message, a window opens as expected and I can  
fill in the expected information.  When I then click send the window  
empties and nothing else happens.  I have set horde to the highest  
logging level but nothing shows up in the log that is relevant.  The  
mail never goes out, and doesn't show up in my sent box.
I can send mail from the command line both by using mail, and also by  
telnet to localhost port 25, so I know that smtp is working on the host.
Any suggestions or guidance would be greatly appreciated.

Thank,

--Adam