Ex2007 coexist certificate error

I am in an ex2007 (win2003) & ex2013 (win2012) coexistence situation.

When I installed the ex2013 server, I purchased a new SAN cert, which I applied to both the 2007 & 2013 servers. The 2007 server had an existing SAN cert, which I did not remove.

The 2013 server sees the new cert as Status: Valid, but the 2007 shows it as Invalid.

Also, in the certificates MMC on the ex2007 server, the new cert has the error:

                 This certificate has an nonvalid digital signature

This same cert works fine on the new ex2013 server. 

The old cert expires tomorrow……what’s going to happen to users trying to connect to mailboxes still on the 2007 server?

 Thanks!