Question: Is it possible to send S/MIME encrypted mails using certificates provided by the receiver?

[JayEn84]

One of our customers asked us to S/MIME encrypt the mails we send to them and provided us with only two .crt files. Is it possible to do that? I see the example on https://github.com/PHPMailer/PHPMailer/blob/master/examples/smime_signed_mail.phps and read the issues concerning S/MIME in this project's issue, but the example requires a private key which I do not have. Will it be possible to send send encrypted mails to the customer anyway? Please excuse me if this is rather an S/MIME-related topic...

[Synchro]

Take a look at #1611 – it's not been merged as I'm very wary of breaking DKIM, but you may be able to get it to work.

To sign a message you will need a private key - you can make one using openssl, or corresponding functions in PHP. To encrypt a message you will need a private key, and also the public key of the person you're sending the message to, which may be available from a keyserver somewhere. Note that though it's "non-standard", PGP encryption is generally more popular than S/MIME, at least in my experience.

[JayEn84]

Is DKIM active by default? (I mean the current stable version, not the PR in reference to #1611 ).

[Synchro]

No it's not, but having DKIM work requires incredible attention to detail, and it's very easy to break it as it's very sensitive (by design), so while it may be possible to get S/MIME working in here, It's not a significant target. I'd say that DKIM is generally far more important than S/MIME, in that pretty much every major email provider supports DKIM, but many have no support for S/MIME.
SwiftMailer may have S/MIME support, so you may be better off using that.