With blocking specific Top Level Domains (TLD) you can greatly reduce the amount of Spam your end users are getting.

The TLDs you are already familiar with are .com, .net. and .org, but there are over 1000 TLDs and some of them are known as suppliers of spam. By blocking these offenders, you reduce the amount of spam making it to your mailbox. There is a tradeoff of course; if legitimate mail comes from one of these TLDs, it will also get blocked, but there is a way around that problem

For more detailed information on which TLDs to block, look here – https://www.spamhaus.org/statistics/tlds/

Only 5 Steps 

Step 1: You need an Office 365 Admin account to do this – Logon to Office 365

After logging on to Office 365, go to the Admin Console, then the Admin Center and finally select “Exchange”

Step 2: Once in Exchange, select “Mail Flow”

Expand Once in Exchange, select Mail Flow – We’re going to create a Mail Flow Rule. Select the “+” to create a New Rule.

Step 3: Creating The Rule

Give your Rule a name and make it descriptive like “Blocked TLDs”. Follow the settings shown in the image

Give your Rule a name and make it descriptive like “Blocked TLDs”.
Follow the settings shown in the image.

Please remember each TLD must have a “$” at the end, no other punctuation is needed as MS adds the rest.
*Optional – you can add other conditions, actions or exceptions at this point if you need to.

Step 4: Final Step

Once you have added all your TLDs, save your new rule, then make sure it is located in the appropriate position on the list

Just like firewall rules, mail flow rules are applied in order. If there is only one rule then you have nothing to worry about.

Step 5: Notice to Admins

Once finished you will receive an email from the Postmaster when an email is blocked. You can monitor these at your inbox and inspect the message for false positives. The image shown is just a snippet of the actual email that you will receive.

You will never be able to stop all spam but you can certainly slow it way down and Office 365 provide several tools for you to customize your own mail rules that will work in your environment.