AT&T has started cracking down on unauthorized tethering, sending out emails and text messages to users it suspects are using the service without paying for it.

Tethering is the practice of sharing a phone's Internet connection with other wireless devices.

AT&T Free Msg: Did you know tethering your Smartphone to a computer requires a tethering plan? Pls call 888-860-6789 for details or visit att.com/dataplans, one text message reads.

AT&T is also sending out email messages to customers, informing them that their service plans may need updating.

The company recommends that users subscribe to its Data Pro 4GB Tethering Plan, which, at $45 a month, combines the smartphone data plan with the tether feature.

Subscribers who do not comply by March 27 will be automatically enrolled in the tethering plan, which will replace their current mobile plan, AT&T says.

AT&T currently offers tethering to iPhone users via the device's Mobile Hotspot feature. With the feature, iPhone owners can share their internet connection with up to three devices via a Wi-Fi.

Before the update, many iPhone users purchased an app called MyWi, which offers the same functionality of AT&T's offering without the extra monthly fees.  Developed by Cydia, the app, which works on jail broken Phones, costs $19.99.

It's still unknown how AT&T determines which users it suspects of unlawful tethering, though it is likely that the company is simply targeting the users with the highest data usage. Many of these users subscribe to AT&T's unlimited data plan, which the company began phasing out last year.