There are many reasons why this could be happening. Below we will explore some of them and offer you suggestions what to do in every case.
Let us start from the most common one: “Email forwarding”
Email forwarding rules are a bad idea
If you have configured the email forwarding feature, to automatically forward all received messages to other services like Gmail, Yahoo, etc, then it is almost certain that at some point you will not be able to receive messages from other people. The senders will get an error message similar to this:
SMTP error from remote mail server after end of data:
550 “This mail cannot be forwarded because it was detected as spam.”
This is quite tricky because the error message is actually coming from the Gmail/Yahoo/etc mail server, see why… The sender is sending YOU a message, YOUR server gets the message, knows it needs to forward it to another service and it tries to do that, but the other service (be it Yahoo, Outlook, etc) rejects this message. The other service will inform YOUR server about the failure and YOUR server will inform the sender, with an error message like the above.
To be on the safe side, do not set any auto-forwarding rules. If you want to get your emails to another services (like Outlook, Yahoo, etc), ask the sender to send their messages directly to your other email address(es).
Mailbox is full
If your mailbox is full, you will not be able to send or receive messages. To check the usage of the space allocated to your mailbox, just login via the Webmail interface. After you login, have a look at the top bar of the Webmail interface. If you see any red numbers (like 999MB / 1000MB) it means that your mailbox is full or it will be full soon. The first number shows the current usage and the second number shows the total space of your mailbox.
If your mailbox is indeed full, follow our guide on how to make some room. You will not be able to send or receive messages until your mailbox has some free space.