Hey there S107
Great question! I run a web-hosting and email server with hundreds of accounts on it, and it seems there is still a mystery about how the darn stuff works. I may use your question as a intro to a whole "email how-to" on the radio if that's OK...
Now - in your case, I will assume that you are using the "add sender to blocked" feature that is common with many email clients such as Outlook, Outlook Express, Thunderbird, and some webmail clients etc... If you are talking about something else (such as SpamAssassin) then let me know what it is and we can re-gig your answer.
First of all - lets note for the record that SPAM is a tough thing to deal with. I mean REALLY tough. The
buggers people that use it are clever, resourceful and well paid.
When an email is sent to you, it contains various information hidden in the "headers". There is lots of stuff, and one of the fields that you would use regularly is the "reply address" or "sender". This is the bit that is displayed when someone sends you an email so that you know who sent it and it allows you to create an email to reply to without having to type in their address.
Here is the catch - most of the headers (data) in an email can be changed by the real sender to whatever they like. This is not a fault or weakness, but a deliberate feature! It means that I can send email when I am out of the office from my phone, gmail or whatever and have the reply address as my correct email address for the office. Nice one eh?
Now -
the problem. Spammers know that if they send email from one "source" address, then they will eventually be blocked - so they just randomise and keep changing the "senders" address and sending your the same spam message... This means that if you block one sender - your email program will automatically filter any other emails from that sender - but they will just change the sender and get through again. Frustrating eh ?
The solution - basically "blocking senders" using your email program is a very very basic method of getting rid of spam - one that will only catch so few spammers that I have not bothered with it for years. It can be helpful if an annoying colleague or family member sends you unwanted jokes & emails etc..
but don't tell them I said that! What you need is a full anti-spam software package that allows you to "test" for spam using a number of techniques.
These sorts of software packages use a number of tests - such as the sender's email address (if by change it is on a known "blacklist") but also checks for keywords in the content, the percentage of images to words etc. etc... They are very very sophisticated, and are configurable so that you can turn the sensitivity up or down. Most of these packages also are maintained and updated so that as spammers evolve new techniques, the good-guys are also improving theirs.
Now you can install a number of products on your local computer - however some of the damage is already done - you will have to download the spam from your web-based mail server, filter it locally and then delete or trash the positive (spam) results. Lots of industry grade security packages have these features, however you will usually have to pay for them to get a decent one.
The best solution is to use a mail server that has an anti-spam package running on the internet - so your mail is filtered before you even get to download it. This is much harder for you to do yourself, but you can shop around and use an email service that provides this level of protection.
Many hosting or mail providers use these services to differentiate themselves in the market - so ask your host questions and vote with your feet (or wallet!) to move to a mail server that is actively fighting the anti-spam war.
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