Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
Quick Guide on Using Personal Certificates in E-Mails
Before we begin, let us first understand what a Personal Certificate is. In simpler terms it is an electronic document that leaves a digital stamp of association with information such as your name or email id. A personal certificate comes handy when you want to send or receive encrypted messages with your signature.
Here are the steps that you need to follow to get personal certificates in emails:
Get the certificate issued: Of course without a personal certificate, you can’t include one. For this, you need to contact certificate authorities such as Thawte or VeriSign for each email address you want to receive or send personal certificate in emails. You may alternatively use Keychain Access, which is included in the Mac OS X.
Create a signed certificate: Once you have got a certificate made, import it in Keychain Access. The following formats are supported by Keychain Accesss: .cer, .crt, .p7c, .p12 or else the program won’t be able to import the file.
If you are not able to import the file, then the certificate must be invalid or might have expired. In this case, contact the authorization authority. To see whether the certificate has been successfully installed or not, check in My Certificate category in Keychain Access.
Using The Personal Certificate: To use the personal certificate, click open the certificate in Keychain Access and ensure that ‘Always Trust’ setting is selected. Once done, the certificate is ready to be sent.
If you want to use the certificate on some other computer, you can copy the certificate file or simply download it from the authority certification company. Repeat the same procedure of opening the certificate in Keychain Access and clicking on ‘Always Trust’.