Why do ISPs recognize messages as spam differently?

The exact algorithms are the ISPs’ and mailbox providers’ “secret sauce” which is not publicly available. However, we know what that spam filters are checking:

• If the sending domain or IP is blocked by any reliable blocklist operator
• If there are any blocked domains within the message content or in a from-field
• If the message content contains any suspicious phrases
• If specific sender has hit any spamtraps recently
• If specific sender sent messages to non-existent addresses recently
• If similar messages were reported as spam by other users recently.

Moreover, ISPs may check individual preferences of the recipient and filter message that is properly delivered to the majority of the list, because
– The specific recipient used to mark similar messages as spam in the past
– The specific recipient ignores a sender (didn’t open messages from the sender for a long time) etc.

Currently, each and every ISP has some kind of anti-spam software in place, but there is no one standard for all email clients. This is what may cause such differences between email clients. The same message sent to two recipients using the same mailbox provider may be delivered properly for one subscriber and be filtered to the Spam folder for other subscribers. The individual preferences of subscribers (based on their past activities) may also cause such a situation.