I have just set up thunderbird for two accounts.  One is an AOL
account and it works just fine sending and receiving emails.  The
gmail account can receive emails by clicking on "check mail" but I'm
unable to send.  When I attempt to send a message I receive the
following error message.

Send Message Error
Sending of message failed.
The message could not be sent because connecting to SMTP server
SMTP.GMAIL.COM failed.  The server may be unavailable or is refusing
SMTP connections.  Please verify that your SMTP server setting is
correct and try again, or else contact your network administrator.