Eircom says all its eircom-dot-net email users have had their access fully restored.
It followed a so-called "intrusion" yesterday, which affected access to the email service.
In a statement, Eircom says there's no evidence to suggest any impact to other systems or services, including customer data.
It has launched investigations into the cause of the intrusion.
Eircom is advising customers to change their email account passwords today - and on a regular basis.