I can't forward a letter

I get an error message:

‘my email address’ not accepted with reason:
4.3.1 Insufficient systrem storage

Hi, seems like your email server has exceeded the maximum size of your mailbox, please check with your mail service provider on how to get more space for your mailbox or archive some of your messages to your eM Client.

You can also clear out space on your mail server by deleting some of the old conversations.

Hope this helps,
Paul