One of the features of SpamFilter is that, whenever an email is blocked, it forces the *remote* server to send the bounce NDR (non-deliverable) email back to the sender to inform them that the email was blocked. This prevents your own network from becoming a source of spam, as most bounces are sent to fake addresses.

SpamFilter does this by rejecting the incoming email with an SMTP error code and a brief description of why the email was rejected. The remote server, seeing the SMTP error code, will then send a bounce email back to the sender, usually including the "reason" of why it was blocked. This "reason" will usually contain the above "brief description of why the email was rejected".

In SpamFilter, you can customize these "brief description of why the email was rejected" for the most common reasons of why SpamFilter rejects an email. You can find these descriptions under the "Settings - Customized Items" tab. Please ensure that you do not alter any of the numeric codes in these entries unless you are 100% sure of the effect this would have.

Also please note that when an email is blocked as spam, it is effectively "not delivered". Unless a user is expecting a specific email, most users will not browse their quarantine for missed emails. The whole point of a spam filtering solution is to relieve users of the burden of having to go thru their spam emails, so checking the quarantine every day is not something a "normal" user would do. And even if they do, as you said it may take days before they realize an valid emais blocked. If an email is blocked, the sender must thus be notified immediately, as the email may be time-sensitive and if the sender doesn't know it was not delivered, and the recipient doesn't check their quarantine, this could cause some very serious problems.