A change to the terms and conditions of Yahoo's free webmail service highlighted the fact the tech giant will "scan and analyse all incoming and outgoing communications content sent and received" from accounts such as Mail and Messenger including instant messages and SMS messages
Furthermore, Yahoo is making users of the webmail service responsible for alerting non-Yahoo users about the potential snooping.
"If you consent to this ATOS and communicate with non-Yahoo! users using the Services, you are responsible for notifying those users about this feature," the Ts&Cs state.
However, Yahoo said the change to the T&C's is simply asking customers if an automated, computer scan, which is currently used to flag-up spam - a process currently employed by a number of webmail services such as Google’s Gmail - can be used to highlight other aspects of the email that could enhance services.
"In June this year, Yahoo began to offer our users in the UK the chance to sign up to our New Mail service. New Yahoo Mail offers users a faster, more social and safer experience, with quicker download of mail, access to your favourite social networks via your mailbox and enhanced protection against spam, among many new and exciting features," the company said in a statement.
"We think transparency is key because our business depends almost entirely on the trust of our users. We therefore ask users (via a pop-up notice) for consent to the extension of machine-scanning inbound and outbound emails to look for keywords and links to further protect you from spam, surface photos and in time, serve users with interest-based advertising," the company said.
"If you prefer not to consent, you can remain on our existing mail, although we will, as Yahoo and other free webmail service providers do, continue to machine scan emails to protect against spam. Last month, Yahoo blocked 550 million inbound spam emails."
Furthermore, the tech firm said it is "committed to protecting users’ privacy" and users could opt-out of interest-based advertising by visiting a dedicated web page.