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SMTP error in Thunderbird

I have a mail server running on a Fedora 6 machine.
We are using Thunderbird as our client mail all the while. It is running well. But lately, we can't send mail out and there is error saying

"An error occured sending mail: The mail server sent an incorrect greeting: Cannot connect to SMTP server XX.XX.X.XX(XX.XX.XX.XX:25), connect error 10061"

I have no clue why suddenly I can't send mail from the thunderbird.
I have squirrelmail client which configure to use sendmail to send the email, it is working well. So, this means that my mail server is functioning well. Only that the thunderbird can't send mail.

Please help.

Thank you.

You haven't been clear of what you run where, but given that error the first check I'd do is for client and serverside firewalling blocking access. If you can telnet from the client to the mailserver port 25 then it's not that.

Thanks for guiding.

I tried telnet XX.com 25

The results I get is as below:

Quote:

Trying XX.XX.XX.XX...
Connected to xxx.com (XX.XX.XX.XX).
Escape character is '^]'.
220 XX.com ESMTP Sendmail 8.13.8/8.13.8; Fri, 9 May 2008 20:57:01 +0800

I have checked all my firewall, I have enable port 25 and my router has enable port 25 also.

Is there anything that I am still missing.

My sendmail.mc is as below:
But it is running well last time. So I didn't suspect it is error from here.

Quote:

include(`/usr/share/sendmail-cf/m4/cf.m4')dnl
VERSIONID(`setup for linux')dnl
OSTYPE(`linux')dnl

define(`confDEF_USER_ID', ``8:12'')dnl

define(`confTO_CONNECT', `1m')dnl
define(`confTRY_NULL_MX_LIST', `True')dnl
define(`confDONT_PROBE_INTERFACES', `True')dnl
define(`PROCMAIL_MAILER_PATH', `/usr/bin/procmail')dnl
define(`ALIAS_FILE', `/etc/aliases')dnl
define(`STATUS_FILE', `/var/log/mail/statistics')dnl
define(`UUCP_MAILER_MAX', `2000000')dnl
define(`confUSERDB_SPEC', `/etc/mail/userdb.db')dnl
define(`confPRIVACY_FLAGS', `authwarnings,novrfy,noexpn,restrictqrun')dnl
define(`confAUTH_OPTIONS', `A')dnl

TRUST_AUTH_MECH(`PLAIN LOGIN')dnl
define(`confAUTH_MECHANISMS', `PLAIN LOGIN')dnl

define(`confTO_IDENT', `0')dnl
dnl FEATURE(delay_checks)dnl
FEATURE(`no_default_msa', `dnl')dnl
FEATURE(`smrsh', `/usr/sbin/smrsh')dnl
FEATURE(`mailertable', `hash -o /etc/mail/mailertable.db')dnl
FEATURE(`virtusertable', `hash -o /etc/mail/virtusertable.db')dnl
FEATURE(redirect)dnl
FEATURE(always_add_domain)dnl
FEATURE(use_cw_file)dnl
FEATURE(use_ct_file)dnl

DAEMON_OPTIONS(`Name=MTA-v4, Family=inet, Name=MTA-v6, Family=inet6')dnl

FEATURE(`accept_unresolvable_domains')dnl

LOCAL_DOMAIN(`XX.com')dnl

MAILER(smtp)dnl
MAILER(procmail)dnl

MAILER(local)
MAILER(cyrus)
define(`confLOCAL_MAILER',`cyrus')
LOCAL_RULE_0
R$=N $: $#local $: $1
R$=N < @ $=w . > $: $#local $: $1
Rbb + $+ < @ $=w . > $#cyrusbb $: $1

If you can connect to the port (remote) and Squirrelmail works (local) then I doubts it's a Sendmail-related problem (check the logs to be certain it's not auth stuff). You said "lately, we can't send mail out". Does this mean you could before? Does the problem apply to all remote clients or some? Different e-mail clients? When did this start happening? Can you trace any configuration changes or updates back to that point in time?