Eir is set to increase the monthly charge for the eircom.net email service to €9.99 from the start of July.
The email service had been provided free of charge for the best part of two decades, before a monthly fee of €5.99 was introduced last year.
In a statement, Eir said the price is being increased to "maintain the service going forward".
"If customers do not wish to pay the increased charge they can cancel by logging into their webmail management page and selecting cancel," the statement said.
"Service users can visit eir.ie/support/webmail to learn more," it added.