emails do not arrive

Hi,

I am using the free mailpoet plugin in 2 different sites.

Recently I have noticed that the emails seem to have been sent, but people are not receiving them.
As an example the one site is very small, only has 20 or so subscribers. Of those only 4 actually got the emails.
The other did not.

My other site has about 250 subscribers and again most of them are not receiving the emails.

Can you please help me with this.

Thanks,

https://www.ads-software.com/plugins/wysija-newsletters/