Every dominionlending.ca email account has a mailbox quota which you need to be aware of. By default this size is set to 25 GB.
To avoid running into this problem, you'll need to delete emails or move important emails into an archive which is outside of your DLC mailbox. If you've already hit your quota and you're now unable to send or receive emails then it will be required for you to submit a support request for us to help out further.
If you'd like to setup an email archive but are unsure of how to do this, please let us know and we'd be happy to help out with that as well.