In response to this topic (http://www.idnetters.co.uk/forums/index.php/topic,31834.0.html), I made some further enquiries, and here is some information on email quotas, received from IDNet:
QuoteThe quotas are dynamic and change according to various threats at the time. Newly created email addresses default to 3 emails or 3 recipients for the first hour, this will then change to 100 messages or recipients, this is the default quota. The quotas will then be adjusted according to user behaviour (i.e. sending spammy emails, weak passwords, not logging in to mailbox via imap/pop/webmail)
For maximum quota users should:
Set a strong password- This should consist of a minimum of upper and lowercase letters (no words), and numbers (possibly a minimum of 4) . A strong password is indicated by the bar underneath the password turning green within your IDNet customer portal.
Log in to mailbox at least weekly
Not send emails with Spammy characteristics
Have virus & malware protection on all connected machines - regularly updated and scanned
Ensure router is not infected with malware and has latest firmware to prevent such malware