Couple questions:
1. What version of OSTicket is this for?
2. What version of MySQL and PHP are you running?
So here's what I did... I created a ticket via the web interface. I dropped to CLI and grabbed the ticket_id of my new ticket (3291). I then copied the code from your post and replaced the ticket_id with my own and ran the query. The query worked fine for me.
INSERT INTO ost_ticket_message SET created=NOW() ,ticket_id=3291,messageId='',message='rnrnrnrnchage control test rnrnrnRob McGeown| Service Delivery Manager| | DDI: 01293 641735 | Mob: 07803 015390rnrnrnEmail Disclaimer: http://www.bytes.co.uk/email-disclaimerrnrn ',headers='Received: from mail pickup service by ukxclexc01.bytes.co.uk with Microsoft SMTPSVC; Fri, 28 Sep 2012 09 +0100rnMIME-Version: 1.0rnContent-Type: multipart/related;rn type="multipart/alternative";rn boundary="----_=_NextPart_001_01CD9D52.FF604739"rnContent-class: urn-classes-MimeOLE: Produced By Microsoft Exchange V6.5rnX-OriginalArrivalTime: 28 Sep 2012 08.0036 (UTC) FILETIME=rnX-RecipFlags: 0rnPrevMsg-ID: rnX-D37DD0F7169A2D7E9FB327ADC88B94C7: InternalrnSubject: chage control testrnDate: Fri, 28 Sep 2012 09 +0100rnPrevMsg-ID: rnX-MS-Has-Attach: yesrnX-MS-TNEF-Correlator: rnThread-Topic: chage control testrnThread-Index: Ac2dUvvcoDpwsytZQpiq6hW19mhNwQ==rnFrom: "Rob McGeown" rnTo: "Change Control" rnrn',source='Email',ip_address='10.0.0.111'
Affected rows: 1
Time: 0.139ms