It sounds like your ISP shutdown port 25. Are you sure it is still operational? Since you didn't make any changes, check with your ISP. If they didn't shut the port down, they may have blocked it in some way (e.g. security based IP filter).

HTH.

>Hi,
>
>We have a Windows NT 4 server with Backoffice SBS 4.5. With have an ISDN connection to an ISP provider. We use it to have access to Internet and to receive (POP3) and send (SMTP) e-amails. We use for that a modem conection and Proxy Server 2.0 for the conection to Internet and Exchange Server 5.5. to control the mailing.
>
>Till last Wednesday everything worked perpectly with the conection. But since that day we're not able to send e-mails. We receive a "Host Unreacheble" message in the SMTP connector of Exchange Server. Also in Events View we have "an Attempt to connect to host failed" message.
>
>We've tried the connection to the ISP trought a Windows'98 PC and everything works perfectly (so we image that is not an ISP problem).
>
>We do a ping to the SMTP server and it works ok. But we try to do a Telnel to the port 25 of the SMTP Server and it doesn't work.
>
>Can anybody help us or tell us where can we found any kinf of help?
>
>Thank you very much in advance.
>
>Juan.