It’s not unusual for Oxford University students to receive unsolicited emails from ‘volunteers’ offering free plagiarism checking services. However tempting this may sound, sending your essay to anyone outside the university can lead to you being accused of plagiarism even though it is your own original work.
Plagiarism checking, proofreading and editing services may be fronts for professional essay writing services and ‘essay mills’. If you send your essay to them and it is sold on and submitted first by another student it will come up ‘100% matching’ when your tutor runs it though Oxford’s Turnitin service. Turnitin checks against essays submitted by other students in other universities all over the world. Turnitin also checks against example essays found on essay mill sites, so if yours has ended up there it will be found on the web even if no-one else has bought it. In these scenarios it would be hard to prove that you are the original author.
If you want to check your work for accidental plagiariam, discuss it with your tutor. Oxford University offers Turnitin via WebLearn, it’s free and easy for your tutor, supervisor or departmental administrator to use.