What is a "digital signature" of emails and what is the difference between a cryptographic signature and an auto-signature at the end of a message?

In everyday speech, the expression "email signature" is often understood as a simple text block or image with contact details that are automatically added to the very bottom of each outgoing message. However, in the context of information security and professional IT standards, a digital signature has a completely different, deep cryptographic meaning. Understanding this difference is critically important for protecting corporate correspondence from forgery.

A traditional text auto-signature carries no protective function and is created manually by the user in the email client settings. It can contain a name, title, social media links, and disclaimers. Any recipient sees this block, but it in no way guarantees that the email was actually sent by the specified person, since the signature text can be easily copied or forged during manual forwarding.

A cryptographic digital signature works based on asymmetric encryption using a key pair. The sender signs the message with their private key, and the recipient verifies authenticity using the sender's public key. Such a signature solves two fundamental tasks at once: it confirms the authorship of a specific person or domain and guarantees that the email text was not modified by attackers during network delivery.

Modern email clients and protocols support automatic verification of such signatures at the infrastructure level. For example, the DKIM technology signs the headers and body of an email on the sender's server side, protecting the domain's reputation. For personal emails of high confidentiality, standards like S/MIME or PGP are used, which are embedded into email programs and allow signing each individual message with a personal security certificate.