The exchange of secure e-mails is based on digital certificates used for both signature and encryption. This means that the certificates practically act as digital passports.
The digital certificate can be used to send electronically signed e-mails. Any modifications in transit can be detected, and the origin of the e-mail can be unambiguously identified. This guarantees e-mail integrity and authenticity.
Encryption is used to send confidential information on a secure basis. Again, the recipient's certificate (public key) is required. Therefore, the public keys must be exchanged prior to the first use of encryption. In the case of S/MIME, the exchange of signed e-mails is usually sufficient. The digital certificate will be available in the e-mail program after automatic or manual import. For PGP, the public PGP key must be sent as an e-mail attachment.