Notification system patent infringement

Hello all,
I am getting emails from some FMCG [email protected]
Regarding their patent infringement by me.
They are saying that i am using their push notification technology which is patent to them and i need to give them the royalty in return, otherwise they will inform the app stores about this.

But the funny thing is that i am not using push notification in either of my apps.
Have any of you have received the email similar to this?

Below is the email received from them.

Dear developer:

According to our previous notification were we inform you about the requirement to legalize your App patent infringement we want to remember you that in the next days will end the term given to accept the two parts agreement with the payment of the conformity royalty.

In the case you don?t regulariza the situation we inform you that we will protect our rights and we will be free to proceed with the next actions:

Your Patent infraction will be formally notified to the different platforms (Apple, Google and Windows Phone). It will be demanded, in accordance with the law, that they stop offering your Application in their respective European platforms.
Loss of the price of conformity Royalty. (The price of the royalty to be paid following the set period will be established according to the number of users, the platforms and the countries in which the App is on offer. Calculate it)
The implementation of any measures necessary to prevent the continuing violation of the Patent.
The publication of the judgement against the infringer at the expense of the guilty party.
If you do have any doubt about it we recommend you to ask for legal advice.
In the event of non-compliance with the commitments set out, we will undertake the appropriate legal action against you before a Court of Law.

They have some similar post talking about this on Reddit /androiddev