Blocking Spam Emails beginning with "@a.*.com"

I have been hammered with Spam email which comes from an email domain beginning with "@a.xxxxx.com"
Despite adding every single domain which begins with "@a." to my blocked domain list, new ones keep showing up.

Unfortunately, Hotmail, Windows Live Mail, and Outlook.com email settings do not allow you to add wildcards (such as "*" or "?") to block senders or create email rules.

To get around this, you can set up a rule to block emails which contain the "@a." text.
This is really straightforward. Simply sign into Hotmail or Outlook.com and navigate to the gear icon.

Gear Icon > More Email Settings > Rules for Sorting New Messages > New

Then fill out the two step rule as follows:

Step 1: Sender's Address > contains > @a.
Step 2: Move to Junk

This should help you get rid of the @a.xxxx.com spam emails.