Sometimes it takes too long to receive an email

Sometimes it takes too long to receive an email

Hello,

I have the problem that sometimes the email for me is simply not delivered, or comes only extremely delayed.
A year ago I got the email 5 days later and this Friday I had to get an important email in time, but it is still not there.
Is this a known problem? Is it me or Zoho Mail or the sender?

Best regards