I remember receiving the first bunch in the late '90s, and the start of the new millenium. If I'm not mistaken, the first batch of email addresses was harvested from Microsoft Hotmail's website, where they created default profiles for email users and associated it with their country, as set during the registration process. But that wasn't an easy way to collect the emails, and at the time, wasn't worth the trouble, as companies didn't see the benefit of spam, yet.
A few months later, things evolved and people started harvesting emails from email forwards. Whenever someone sends an email, they stuff all the emails into the "TO" field, and off they send it. When the recipient clicks on the forward button to forward the message, the header of the old message is dumped as part of the new message (the forward), containing the list of emails of all the people to whom it was sent to previously.
I know 4-5 people who actually spend time to clean the email list, before sending a forward. And the same 4-5 people, use the "BCC" field, instead of the "TO" field. Whomever receives the emails, doesn't see the list of emails.
Unfortunately, this continues to this day, and emails are still being collected and spammed, because of 2 things: E-Mail providers are too lazy to filter out list of emails, and people are too lazy to do the same!!!
If you're wondering why this is specific to Arabic Spam only, it's because non-Arab sites spam using Spamming services! They send emails using brute-force and don't actually care whether you're an Arab, or not.