You get reports that email being sent to you is not being delivered.
Ask the sender to send you a copy of the bounce back massage they get, to a different email address of course.
Look down towards the bottom of it, often the last line, and see if it says:
maildrop: maildir over quota.
If it does then your server email box is full and reached its quota.
To resolve this you can:
If connected via POP
Log in to http://webmail.truska.net using your username and password and start to delete messages from the server
All your email should already be stored locally so you should not loose anything - but check with your admin if you are not sure.
You can also check your Outlook (or other client) settings to make sure that emails are been deleted off the server automatically in a timely manor (Outlook | Account Settings | More | Advanced)
If IMAP - where we store your email for you - you will need to do some house keeping by filing older emails into local folders, by deleting old emails, clearing out your Junk and/or Deleted Items Folder etc. You could also remove large attachments from emails.
You can of course purchase additional space in 1GB lots - call or email support to action this.
General housekeeping should be done on all email boxes periodically anyway by archiving and or deleting older emails.