We have a multitude of methods to block spam. Among them is the "greylisting" technique. In this article, you will learn how greylisting works and how this affects you. In short: you may experience incidentally a delayed arrival of your email.
First, it is good to know how a spam-sending server works. Usually, it tries to send a spam email to all users in a list. It does not care about non-existing email addresses or errors. If the delivery fails, it will not make a second attempt.
In a normal case, the sending server of a legitimate server contacts our servers. We validate the request and if all criteria are well, we will process the email. However, if some important criteria are not met, the server is told to deliver the email later again. This is called "greylisting". We are not blocking the server, but simply asking to send it later again. Once it does, the email of this server does not have to pass the greylisting method for some time. This means that emails that are sent later will arrive immediately. Also, we have whitelisted the big email providers for your convenience.
This is the difference between most spam-sending servers. They do not attempt to send the email again and will just ignore this. Legitimate servers will make a second attempt and succeed.
Do note that some spam-sending servers do support retries, but we have other tricks to block that kind of spam as well.