I'd really like to be able to let smime.p7s through the gateway.

Hello Shawn,

Thank you for contacting the Sophos Community!

What is the reason in Sophos Central email currently, that the emails with p7s are getting quarantined?

Regards,

Central is using True-File-Type Detection.

So actually the P7S will be looked at the real filetype is a certificate. See: https://stackoverflow.com/questions/21056781/how-to-get-mime-type-of-p7s-files

Its not actually p7s, instead it is using .der. So thats the reason, Central is not listing p7s. If you use a DLP rule for the cert types like der, it should hit by p7s files. 

Central is using True-File-Type Detection.

So actually the P7S will be looked at the real filetype is a certificate. See: https://stackoverflow.com/questions/21056781/how-to-get-mime-type-of-p7s-files

Its not actually p7s, instead it is using .der. So thats the reason, Central is not listing p7s. If you use a DLP rule for the cert types like der, it should hit by p7s files. 

Thank you Tony,

IMO If Sophos is going to call it P7s then it should be in the list as P7s.

Here is the current "Use Sophos list(recommended)" Certificates list, only .crt  is check marked. .der is not.

.arm
.cer
.cert
.crt √
.der
.p12
.pem
.pfx