STARTTLS Exchange certificate will expire soon

My Exchange 2010 server has 3 certificates listed.  There are two self signed certs that have the SMTP service assigned, and one from a certificate authority that has IMAP, POP, IIS, and SMTP assigned.  We've recently started getting errors in the event viewer that say:

The STARTTLS certificate will expire soon: subject:, thumbprint: ABCDEFGHIJKLMNOPQRSTUVWXYZ, hours remaining, 645.  Run the New-ExchangeCertificate cmdlet to create a new certificate.
I have several questions about this.  First of all, why do I have 2 self signed certs that both have SMTP assigned and expire within days of each other?  Why is only one of the two self signed certs (the one that expires latest) being alerted on in event viewer?  And do I really need either of these, since the cert from the CA has SMTP assigned to it?
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IvanSystem EngineerCommented:

when ever you deploy Exchange, you get one self signed cert, by default.
If you deploy new cert, from either public CA or Internal CA (like in your case) and bind all services, then you don't need to renew self signed cert.
Why do you have 2 self idea :)

PS: To remove cert:
Remove-ExchangeCertificate -thumbprint “ABCDEFGHIJKLMNOPQRSTUVWXYZ"


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