To apply the patch:
Unzip the patch files somewhere, in the includes/ directly might be easier for you.
unzip osticket.1.6.rc4.patch.for.email.zip
cd to the directory to where your include/ directory is.
cd /path/to/osticket/include
You'll see the files you need to patch in there:
ls -al
class.validator.php
class.pop3.php
class.ticket.php
Now patch your files:
patch -p1 < /path/to/my/patch/you/downloaded/class.validator.php.patch
patch -p1 < /path/to/my/patch/you/downloaded/class.pop3.php.patch
patch -p1 < /path/to/my/patch/you/downloaded/class.ticket.php.patch
It may ask you to verify which file you are patching, since we're in that directory you can just specify the file name.
Now you're done, you should see the changes work immediately.
I had one problem with the patched files, now the mysql tables didn't match any longer and I received the following admin error mails:
[INSERT INTO ost_ticket SET created=NOW() ,ticketID=726996,message_id='
- Unknown column 'message_id' in 'where clause'
To fix this I told MySQL to add the missing columns:
alter table ost_ticket add column message_id varchar(255)
alter table ost_ticket_message add column message_id varchar(255)
Send a test email and it should all work perfectly. Note that I didn't check what kind in input message_id is, the message_id looks something like so you might want to be more specific.