Unable to send and to receive mails (qmail) since today

Its been some time since I've used qmail...anyway we'll give it a try
are you sending mails directly or are you relaying through another server ?

if its the former, try manually telnet to the port 25 of the mx server of one of the domains like fedex.net from the qmail server and try sending a mail
in case you need here's the reference for trying to send mail via telnet.
http://www.rdpslides.com/webresources/FAQ00035.htm

this way it can be isolated to a system level issue ( like dns not reachable, firewall problems etc) or a qmail configuration issue.

Hello
Thank you for your answer.
Here is my data:
Internally we use directly our mailserver. Externaly we use a mailscanner (greylisting), but this mailscanner get's the same error message we get when we telnet localhost 25 (see below).

Try to use another mailserver gives this result:
---------
telnet smtp.hispeed.ch 25
Trying 213.46.255.24...
Connected to smtp.hispeed.ch.
Escape character is '^]'.
220 smtp.hispeed.ch ESMTP server (InterMail vM.7.08.02.00 201-2186-121-20061213)                                            ready Wed, 23 Jul 2008 23:20:03 +0200
ehlo mailhost.foo.bar
250-smtp.hispeed.ch
250-HELP
250-XREMOTEQUEUE
250-ETRN
250-AUTH=LOGIN PLAIN
250-AUTH LOGIN PLAIN
250-PIPELINING
250-DSN
250-8BITMIME
250-STARTTLS
250 SIZE 15360000
mail from:reto.buchli@foo.bar
250 Sender  Ok
rcpt to:xxx@hispeed.ch
250 Recipient  Ok
data
354 Ok Send data ending with .

Testdata from WSL
.
250 Message received: 20080723212252.IEAH21702.viefep23-int.chello.at@mailhost.foo.bar
quit
221 smtp.hispeed.ch ESMTP server closing connection
Connection closed by foreign host.
------------

This works. We already checked our DNS and network switches. They seem to be ok.

Now the same with our mailhost: (this we tried already, but you will see the client answer):

-------------------
telnet localhost 25
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 mailbi.wsl.ch ESMTP
ehlo localhost
250-mailbi.wsl.ch
250-PIPELINING
250-SIZE 40000000
250-DATAZ
250-STARTTLS
250 8BITMIME
mail from:reto.buchli@wsl.ch
250 ok
rcpt to:reto.buchli@wsl.ch
250 ok
data
354 go ahead punk, make my day

Testdata from WSL
.
451 qq temporary problem (#4.3.0)
quit
-------------------

This fails. It is the same error message, we get sending a mail internally.

Now, what may be wrong here?

Thank you.

Good work Reto..

Sorry I couldn't be of much help...its been two years since I've touched qmail :(