I used to be a product manager in this space.

Common explanations for these charges are as follows.

When originating a SMS (i.e. not reponding to a mobile number) users select names from their contact address book.  Increasingly today address books are populated from the likes of facebook with multiple entries for the same person, some of which will have an email loaded but not a mobile number.

Users select addressess by name and effectively load their email address as the destination.

The handset has a shortcode coded into it for SMS to email relay, in the case of Telecom 2428 "CHAT" as I recall (quick chat once used this service). When it arrives at the network the destination is recognised as an email and the SMS to email replay is engaged.  Its not an "MMS" but its the same tariff of 20 cents.

Another scenario is forwarding emails to a contact but SMS rather than the email address is loaded as the destination.  The email to SMS relay is engaged, again 20 cents.