SwissCenter does not use greylisting: this anti-spam technique, simple but rudimentary, significantly slows down the delivery of legitimate emails.
In short: no, SwissCenter does not apply greylisting, which temporarily refuses (4xx code) the first message of each sending server, sender and recipient triplet until it is resent, which delays the delivery of legitimate emails.
What is greylisting?
Greylisting is a very simple anti-spam technique that consists of temporarily rejecting a message by returning a temporary refusal code to the sending server. In most cases, sending servers resend the email after a few minutes.
How does greylisting work?
Greylisting works as follows:
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For each email received, a triplet is created, identified by the sending server's IP address, the sender's email address, and the recipient's email address.
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If the triplet appears for the first time, the mail server returns a 4xx code (temporary refusal) to the remote SMTP server. If this server is a genuine SMTP server, the email will be resent later.
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If the triplet reappears after a minimum delay, set by the server that applies greylisting, the message is accepted and the triplet is added to a trust list ("whitelist").
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If the email is resent before this delay, it is temporarily refused again.
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Triplets in the greylist that do not reappear are deleted after a few hours, and the trust list is also cleared periodically. These delays vary from one server to another.
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