Server does not support secure connections

Out of the blue suddenly all outgoing email messages are delayed since yesterday. Exchange generates a warning email stating that delevery is delayed. Troubleshooting the Exchange message queue unearths that the error is 'server does not support secure connections' I did not touch any settings recently, the server's third party certificate is valid through 2013.

What is happening?

EDIT: I checked the Certicicate using Get-ExchangeCertificate | fl. I found a few self-issued certificates that were valid, but my comercially issued certificate is qualified as invalid (I removerd the SN en the thumbprint:

AccessRules        : {System.Security.AccessControl.CryptoKeyAccessRule, System
                     .Security.AccessControl.CryptoKeyAccessRule, System.Securi
                     ty.AccessControl.CryptoKeyAccessRule, System.Security.Acce
                     ssControl.CryptoKeyAccessRule}
CertificateDomains : {www.fk27.com, fk27.com} 
HasPrivateKey      : True
IsSelfSigned       : False
Issuer             : CN=EssentialSSL CA, O=COMODO CA Limited, L=Salford, S=Grea
                     ter Manchester, C=GB 
NotAfter           : 20-9-2013 1:59:59 
NotBefore          : 19-9-2008 2:00:00 
PublicKeySize      : 1024 
RootCAType         : Unknown 
SerialNumber       : (removed) 
Services           : IIS, SMTP 
**Status             : Invalid** 
Subject            : CN=www.fk27.com, OU=EssentialSSL, OU=Domain Control Valida
                     ted 
Thumbprint         : (removed) 

So, what do I do now?

Solution 1:

While the Get-ExchangeCertificate | fl command still shows that the Essential SSL certificate is invalid, the sending of email has resumed without errors without my doing anything about it...