Check out this article as the answer is pretty much exactly the same 
ARTICLE: Who should I send Printed Newsletter to?
Although you should only send Postcards to your existing clients if the offer on the particular Postcard is directly relevant to them.
For example, you don’t want to send a Postcard to an Existing Client offering them 2 free hours of I.T. Support if they are already on one of your Fixed Fee MSP plans 
However, you do want to send a Postcard to an Existing Client if it’s promoting one of your Services that they haven’t taken up yet.
(e.g. Telephony/VoIP, Cybersecurity, Compliance etc).
This is why segmenting the data in your CRM is so powerful as it helps you easily pick and choose who to send campaigns to based on how you’ve tagged & categorized people in your database.
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