Certificate errors on Exchange 2007

We have a Exchange 2007 server that is recording certificate errors in the event log (server & domain names changed for post):
Microsoft Exchange could not find a certificate that contains the domain name contoso.com in the personal store on the local computer. Therefore, it is unable to support the STARTTLS SMTP verb for the connector DNS with a FQDN parameter of contoso.com.
Microsoft Exchange could not find a certificate that contains the domain name server.contoso.com in the personal store on the local computer.

I have checked the configuration of the send and receive connectors:

Get-SendConnector | FL name, fqdn, objectClass
Name : DNS
Fqdn : contoso.com
ObjectClass : {top, msExchConnector, mailGateway, msExchRoutingSMTPConnector}

Name : Host IT SMTP
Fqdn : contoso.com
ObjectClass : {top, msExchConnector, mailGateway, msExchRoutingSMTPConnector}


Get-ReceiveConnector | FL name, fqdn, objectClass

Name : Default servername
Fqdn : servername.contoso.com
ObjectClass : {top, msExchSmtpReceiveConnector}

Name : Client servername
Fqdn : servername.contoso.com
ObjectClass : {top, msExchSmtpReceiveConnector}

There is an installed certificate:
{mail2.contoso.com, www.mail2.contoso.com, autodiscover.contoso.com, legacy.contoso.com} - IMAP, POP, IIS, SMTP valid until 09/01/2016

There was a expired certificate:
{servername, servername.contoso.com} - SMTP valid until 08/12/2010

The fact that the mail is still working despite the expired certificate, makes me wonder if I could just change the receive connectors to use mail2.contoso.com instead of servername.contoso.com
In the same vein, could I change the send connector to mail2.contoso.com from contoso.com