smimesign | An S/MIME signing utility for use with Git | TLS library

I have a need to ensure that an SMTP server certificate is signed by a public certificate authority. I would like to use phpseclib or some other trusted library. I believe I can use the root certificates extracted from Firefox.

There are some home-brew approaches here to check cert dates and other metadata, but it does not look like that does any signature checking as such (other than ensuring that OpenSSL does it). In any case, I want to use a library - I want to write as little certificate-handling code as possible, since I am not a cryptographer.

That said, the answers on the above link were still very useful, as it helped me get some code to fetch a certificate from a TLS conversation:

For last few hours I have been trying to send signed mail using swiftmailer via sendgrid SMTP server I'm successful with sending signed text emails but when I try to send html content in email sendgrid changes something and the signature becomes invalid any idea how to dump email before sending to know what is wrong? Or how to fix it?

PS: I have already disabled tracking pixel and it works with amazon.

Mailer config: