thanks again.
unfortunately i do not have the pro version of iredadmin, so setting this up there was not an option.
however, after going through quite a number of scripts, i realised that you did not actually insert these values implicitly, but instead you let the default field value be inserted by the database.
that makes sense.
of course that meant that all i needed to do was to change the default field value and it should set things up the way i wanted and that was the case.
so now all that appears to be working perfectly. i will just have to make sure that i check those values whenever an update is done - that's ok.
then the only thing that was not working was the attachments being stored in my chosen directory through the mail_attachment_dir directive in 10-mail.conf.
after trying different things i realised that 10-mail.conf was probably not being used. i then put the directive in the dovecot.conf file and hey presto - it worked!
so to recap for those people who may want to do a similar thing (please note my configuration at the top of this thread)...
after setting up the mailserver in the normal way i did the update as suggested by zhang, above.
i then changed any mailboxes in the user database to the following field values for the mailbox table:
mailboxformat:sdbox //could be sdbox or mdbox - pros and cons for each - previously maildir
mailboxfolder:dbox //this can be anything - your choice - previously Maildir
i then changed the default value for these two fields to the value i input for the current record(s)
i then made the following changes to /etc/dovecot/dovecot.conf :
//this was previously there but i edited it - not 100% sure it does anything - zhang might like to comment
mail_location = sdbox:%Lh/dbox/
//add - this will put attachments in a file for each domain i manage
mail_attachment_dir = /mnt/email_attach/%d/
i restarted dovecot
# /bin/systemctl restart dovecot.service
it all seems to be working fully now although i am happy to be corrected on any points.
this has the advantage of being able to store the bulk of your email content on a separate disk.
also, as i understand it(and this appears correct), each attachment is checked for its uniqueness, so that only once instance of an attachment is saved to the server (i stand to be corrected on this).
the only thing that i am not 100% certain about are the other instances of where maildir is specified in a number of other files. i took it from zhang's instructions that they did not matter
hope this helps someone else as there is not a lot out there on this type of set up.