See if this helps:
XCLN: Error Message: Messages Can Be Received But Not Sent
Applies To
This article was previously published under Q225538
SYMPTOMS
If you send mail from Pocket Outlook or any POP3 client connecting to
an Exchange Server computer, you may receive the following error
message:

Messages can be received but not sent. SMTP server unavailable.
The text of the message may be different depending on the client
used. For example, using Microsoft Outlook Express version 5.0 the
error message appears as follows:

An unknown error has occurred. Account: 'SMTP', Server: 'Servername',
Protocol: SMTP, Server Response: '421 bogus.domain.com connection
limit reached', Port: 25, Secure(SSL): No, Server Error: 421, Error
Number: 0x800CCC67
The 'SMTP Address' and 'Servername' and 'IP Number' can vary and
depends on your Internet Service Provider (ISP) and your exact email
configuration.

The TCP/IP connection was unexpectedly terminated by your mail
server. If this problem persists, contact your LAN Administrator or
Internet Provider.
CAUSE
The maximum number of inbound connections the Internet Mail Service
(SMTP server) can accept at one time has been reached. The most
precise of the above error messages is from the Outlook Express
client, but be aware that if there are many SMTP clients attempting
to send mail simultaneously, one of the other errors above may be
displayed.
RESOLUTION
The SMTP server runs under the Internet Mail Service in Exchange
Server. To resolve this issue, increase the maximum number of inbound
connections for the Internet Mail Service. This value is set to 30 by
default.

To change this value, perform the following steps:

 1.. Start the Exchange Server Administrator program.
 2.. In the left pane, under the organization name and the
appropriate site, click Connections, and in the right pane,
double-click Internet Mail Service to open its properties.
 3.. Click the Connections tab, and then click Advanced.
 4.. Set the Max No. of Inbound Connections to a higher number, such
as 100. This number may vary; increase it until the behavior is
alleviated.
WORKAROUND
If you are unable to change the settings on the SMTP server, send
your e-mail at another time.
MORE INFORMATION
To reproduce the behavior, set the Max no. of Inbound Connections to
zero. You receive the error message on the Windows CE device every
time you try to send mail.

To verify that you are experiencing this issue, open Windows NT
Performance Monitor and compare the Max No. of Inbound Connections
value on the Internet Mail Service with the following counter:
Object: MSExchangeIMC
Counter: Connections Inbound

If these values are equal or even close, increase the Max No. of
Inbound Connections value on the Internet Mail Service.

Also, neither my  Norton System works or my Zone Alarm is
set to scan emails.  Also the issue happens when Zone
Alarm isn't even running anyway.  For some reason, if I
telnet outgoing.verizon.net I get an error that says it
can't open port 23. Even though the default settings are
using port 25/110.
Pete