How do Zoho and ProtonMail handle unknown users?

Zoho and ProtonMail handle unknown users with some distinctions:

Zoho approach:

Standard 550 responses for invalid users. "550 5.1.1 User unknown." May have stricter rate limits for new senders. Business-focused with spam protection.

ProtonMail approach:

Privacy-focused handling. Standard 550 for non-existent users. May delay responses to prevent enumeration. "550 5.1.1 The email account does not exist."

Common characteristics:

Both follow RFC standards for basic codes. Both implement rate limiting. Both provide less detail than Gmail or Microsoft. Privacy considerations may limit information.

Handling recommendations:

Treat standard 550 5.1.1 as hard bounce. Respect rate limiting signals. Expect less verbose error messages.

Privacy-focused providers reveal less. Standard codes, minimal extra detail.