Plopmail was set up 16 years ago as a free email service using the everyone.net provider. Everyone.Net have decided to stop providing a free service from 23rd March of this year with the facility for sending email finishing on 23rd February.
I can see why they would take this position if it is not a profitable service although it is unfortunate this is happening with little notice.
The plopmail.com service will completely close from 23rd March and I will probably just let the domain name expire, otherwise I am paying for a domain name for Everyone.Net to run a business (I don't receive any payment).
Please could you try to find a suitable email provider to move to? I recommend gmail (https://www.google.com/gmail) or hotmail (https://www.hotmail.com) if you are having trouble finding a provider.
Please do not sign up for the paid Everyone.Net service (mailplus) if offered as it will not function once the domain name expires.
I'm not sure how many people actually use plopmail now, although I am given the figure for the amount of registered users I expect the amount that still use it would be quite a bit smaller now.
I hope you have found the service useful and I wish you all the best for the future.