kmail cannot verify authenticity of email server

[Richard Dawson]

Suddenly kmail is unable to verifiy authenticity of my ISP's email server, 
neither sending nor receiving. When I attempt to receive mail I get a message, 
"The server failed authenticity check (ipostoffice.worldnet.att.net.). The 
certificate cannot be verified for internal reasons."

 I get a similar message if I attempt to send mail. Clicking "Details" on the 
message provides the following information:

Sending:


Current connection is secured with SSL
Address imailhost.worldnet.att.net
ip address 204.127.217.19
Encryption: AES, using 256 bits of a 256 bit key
Details: Auth=RSA, Kx=RSA, MAC = SHA-1
SSL Version: SSLV3
Certificate chain: imailhost.worldnet.att.net
Subject:
Common name: imailhost.worldnet.att.net
Organization: AT&T Services, Inc.
Organizational unit: Worldnet mail (SMTP) 3
Country US
State New Jersey
City Middletown

Trusted No, there were errors: The certificate cannot be verified for internal 
reasons
Validity period: 4/27/09 – 4/2710 11:59 pm
Serial Number
MD5 Digest: 4a544c11f4a2158d655c284938140643

Issuer:
Common name: VeriSign Claass 3 Secure Server CA
Organization VeriSign, Inc.
Organizational Unit: Termos of use at https://www.verisign.com/rpa (c)05
Country: US

Receiving:
Address ipostoffice.worldnet.att.net
IP Address: 204.127.134.145
Encryption AES, using 256 bits of a 256 bit key
Details: Auth=RSA, Kx-RSA, MAC-SHA-1
SSL Version: SSLv3
Certificate Chain: ipostoffice.worldnet.att.net
Common name: ipostoffice.worldnet.att.net
Organization: AT^T Services, Inc.
Organizational unit: Worldnet mail (POP3) 3
Country US
State New Jersey
City: Middletown
Trusted: No, there were errors. The certificate cannot be verified for internal 
reasons
Validity period: 04/27/09 12:00 am to 04/27/10 11:59 pm
Serial number: fedc25038240b9e669d98b4443d47b77

Issuer:
Common name VeriSign Class 3 Secure Server CA
Organization: VeriSign, Inc.

I cannot be absolutely sure, but I think this problem began after the last 
upgrade from Mandriva.

Can anyone suggest what I can do to correct this situation?

Richard
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