How can I be alerted before my mailbox becomes full?

Using a Mailrule entry there are two variables you can test for mailbox size, either MAILBOX which is the size of the mailbox before an email is delivered to an inbox or MAILBOXNEW which is the size of a mailbox after an email arrives in inbox,  the example below will help you implement a test of inbox size and an alert..

EXAMPLE: lets say my mailbox size is set to 100MB by having 100M in the .quota file in the root of the account and I want to be alerted when the mailbox size is > 90M which in bytes translates to 94371840.

enable multimatch 
strip if MAILBOXNEW > 94371840 Subject 
insert if MAILBOXNEW > 94371840 Subject "Mailbox over 90M! {subject}" 
disable multimatch  

Here is a handy javascript based byte converter...:

1 Byte = 8 Bit
1 Kilobyte = 1024 Bytes
1 Megabyte = 1048576 Bytes
1 Gigabyte = 1073741824 Bytes

The Byte Converter by Malte Philipp