Renewing an Exchange Certificate - Self signed / StartTLS or Transport (Exchange 2007 / 2010 / 2013)

source: http://forums.msexchange.org/Renewing_self-signed_SMTP_certificate/m_1800558152/tm.htm

Bharat's article is what I've used numerous times.

Just do a Get-exchangeCertificate | FL

Then with what youve provided you just highlight the SMTP cert
Thumbprint : 15405C99D3837CFF0DD2EA0213DAD6A241B

and then type out

Get-ExchangeCertificate -thumbprint “15405C99D3837CFF0DD2EA0213DAD6A241B” | New-ExchangeCertificate

then just bounce the microsoft exchange transport service.

source: http://forums.msexchange.org/Renewing_self-signed_SMTP_certificate/m_1800558152/tm.htm