Eir is to begin charging tens of thousands of people and organisations €5.99 per month for its Eircom.net email service, which has been free for almost 20 years.
From March 31, Eircom.net account holders who don't pay won't be able to open individual email messages.
If they don't pay within two months, their email account will be deleted.
While the company will not say how many free Eircom.net email addresses it supports, sources close to the company say that it is tens of thousands.
This includes community associations, schools and voluntary associations.
Since their introduction, Eir email services have typically been set up and relied upon by people in areas with little choice but to use Eir for internet and broadband services over the last decade.
Eir is understood to be implementing the move as the email service is a loss-making one that requires customer service resources.
In the UK, telecoms companies have been introducing similar charges on once-free services. BT customers reacted with dismay when it recently raised prices to £7.50 (€8.80) per month for customers to use its email, while TalkTalk charges £5 (€5.88) per month.
The €5.99 charge by Eir is the same price as some popular online streaming services, such as Amazon Prime Video and Disney's upcoming Disney Plus.
Eir executives say they do not make money from email services, unlike Google, which sometimes uses the data from its Gmail to supplement its advertising business.
However, it may leave thousands of people feeling trapped into paying a new monthly subscription to avoid losing access to long-standing business, community and personal contacts.
It may also put more pressure on homeowners' monthly subscription bills, which are already soaring from services such as Netflix, Spotify and iCloud.
Eircom.net email users who do not wish to start paying the new monthly subscription can export their emails and contact addresses to free services such as Gmail. Instructions for doing so can be found at support.google.com and involve setting up a Gmail account or using an existing Gmail account.
A spokesman for Eir said the company will not waive the monthly fee for those who already subscribe to its broadband, phone, mobile or bundled products.
"Eir will introduce a €5.99 per month subscription charge for the Eircom.net email service from March 31, 2020," said the spokesman.
"The charge will be used to invest in the maintenance and improvement of the service going forward.
"If an account holder does not get in touch with Eir to arrange payment, the service will cease after 60 days and the account will be permanently deleted after that time.
"Service users can visit eir.ie/support/webmail to learn more," he added.