Many taxpayers have been receiving this email alert from HMRC :

You’ve got a new message from HMRC

Dear XXXXX

You have a new message from HMRC about Self Assessment.

To view it, sign in to your HMRC online account.

For security reasons, we have not included a link with this email.

Why you got this email

You chose to get paperless notifications instead of letters by post.

This means we send you an email to let you know you have a new message in your account.

From HMRC Self Assessment

In most cases it’s just a reminder that another tax year has ended, and you’ll need to prepare and file a tax return.
These reminders used to be sent by post to self employed taxpayers, but many sole traders have opted to receive email reminders, sent either to their own email address or to their agent’s.
HMRC won’t expose your private tax business in an email, so they offer a Mailbox instead, linked to your Government Gateway log in. Then they post reminders and updates there instead.

So – No need to worry! Though it’s always worth checking on alerts like this. There may be other notifications that aren’t just reminders.