What is 'Entrust' and why do you need it? Thunderbird supports S/MIME natively, no add-ons or 3rd-party software is needed.

I've migrated to a new laptop and imported my certificate.

Along with the cert you'll also have to import the corresponding private key. Did you?

I've had people with whom I exchange encrypted emails on "old" laptop send me digitally signed emails

Then you also do have their certificate.

... but I cannot get their certificates.

You'd need to explain in more detail what that means, and what you're doing.

What is 'Entrust' and why do you need it? Thunderbird supports S/MIME natively, no add-ons or 3rd-party software is needed.

Entrust provides encryption certificates. Our company is required by contract to use Entrust for encryption since we have access to sensitive personal information.

I've migrated to a new laptop and imported my certificate. Along with the cert you'll also have to import the corresponding private key. Did you?

Please explain how I would do that.

I've had people with whom I exchange encrypted emails on "old" laptop send me digitally signed emails Then you also do have their certificate.

When I view the "certificate" of the person I'm testing this out with, Thunderbird shows "not valid". But I do have their valid certificate in my/Thunderbird Certificate Manager.

... but I cannot get their certificates. You'd need to explain in more detail what that means, and what you're doing. I'm trying to send email with end-to-end encryption as I did on my old laptop. Now have new laptop. and Entrust's product support in Germany is not able to help me. I'd like to get assistance from someone who has used Entrust's product (non-enterprise). Thank you for your response.

Entrust provides encryption certificates.

So it is a Certificate Authority (CA).

I've migrated to a new laptop and imported my certificate.

As said before just the certificate won't cut it. You can either export (or backup) your cert on the old laptop. When doing that Thunderbird will prompt you for a passphrase which is supposed to protect the private key. Or you import the cert from a backup you created when it was received from Entrust initially, which includes the private key. Note, Thunderbird would prompt you for the passphrase when importing the cert on the new laptop.

Did you get a passphrase prompt when importing the cert?

I do have their valid certificate in my/Thunderbird Certificate Manager.

I cannot get their certificates

Two statements which contradict each other. Beats me.

When I view the "certificate" of the person I'm testing this out with, Thunderbird shows "not valid".

Please post a screenshot.

Entrust's product support in Germany is not able to help me.

Unless you do need a new cert, I'm not sure how Entrust can help you.