I have read that the api currently does not support modifying tickets. What I'm trying to do is have 'clients' (These are actually automated emails from other systems) post replies to existing tickets.

For instance, I get an email with the subject '#000.23' or something similar, but always the same. This posts to an existing ticket #000.23. Is this possible? If it's not possible with the vanilla installation, is this possible with, say, a python script?

Your automated emails are not replying to the emails. They are sending new ones.
So the normal means of determining if a email belongs to an existing ticket (message-id, etc) will not work.
If you have a way for the automated system to put am osTicket ticket number into the subject you could match on that... but there really isn't a way to do that I can think of.

    ntozier If you have a way for the automated system to put am osTicket ticket number into the subject you could match on that...

    I have tried so far Subject: #000.23 and Subject: 000.23
    Both of these subjects result in a new ticket being formed.

    What Subject would bind it to a ticket?

      Does ticket 000.23 exist in osTicket?

      I figured it out. You actually have to add [#000.23] to the subject. So inside square brackets, with the hash symbol.

      Yes you do.

      [#%{ticket.number}]

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