Over two thirds of surfers don't realise many free email providers scan messages for keywords in a bid to place more user-targeted adverts.
The Attitudes to Email study carried out by email provider GMX found that not only were the majority of people unaware that the practice took place but that one in four said it would affect their next choice of provider.
Only nine per cent were aware of ad-scanning and had considered this when choosing a provider.
Once aware of the issue, over half of Britons (56 per cent) believe that users should be able to opt out of such scanning and 16 per cent were actually anxious at the thought of it.
All email providers automatically scan incoming email traffic in order to identify risks to the user such as spam emails and viruses. However a select few use these scans for ad placements as well.
Managing director of GMX, Eva Heil said: "British consumers should bear in mind that there is a choice of free email services on the market that don't do this. If they are uncomfortable with the practice either for their own or their children's email, they should change provider."