An investigation has been launched after details of hundreds of landlords who have not yet registered on a Wales-wide scheme were revealed.
Rent Smart Wales contacted people who had started but not finished the registration process, but the email addresses of recipients could be seen in the message.
Cardiff council, which handles Rent Smart Wales, said it is aware of an issue and it is being investigated.
Many buy-to-let landlords could face fines and possibly even prosecution for failing to sign up to the new registration and licensing system in Wales.
More than two months have passed since the Welsh government’s Rent Smart Wales scheme became law, and yet it is estimated that thousands of private landlords in Wales have still not signed up to the scheme, which could mean that they are letting out properties illegally.
The email sent out told landlords: "We are writing this email to you as you have a started but not complete landlord registration with Rent Smart Wales."
Douglas Haig, director for Wales for the Residential Landlords Association, said: "We have long been warning of the need for greater security around the Rent Smart scheme to prevent this kind of error occurring.
"With landlords and letting agents expected to register by law, they need to have the confidence that their personal details will be handled sensitively.
"Whilst we are sure this was an innocent mistake and a simple case of human error, we would like to see measures put in place to ensure it cannot happen again."
Property experts at the Cardiff office of Bruton Knowles fear many buy-to-let landlords outside of the principality could also be in line for a nasty shock.
The company claim that the main issue for non-Wales based landlords is the complete lack of awareness that they had to sign up to the new scheme.
Rent Smart Wales (RSW), the new registration and licensing system in Wales, which went live in 2015, represents a major change for the private rental sector in the principality.
The scheme requires all landlords and letting agents to register their properties and undergo training to obtain a licence if they wish to self-manage their rental investment.
Landlords and agents were given until 23 November in which to comply with the new legislation, before it then became an offence to let or manage a property in Wales without the correct licence.
Caroline Jones from Bruton Knowles said: “The enforcement powers under Rent Smart Wales are now active. This means failure to comply with the legislation is an offence.
“However, we know of numerous cases of people living outside of Wales who had no idea on the new Rent Smart scheme. Our fear is that there are many landlords out there who are blissfully unaware of their legal obligation.”
According to Jones, no major pro-active activity was undertaken outside of Wales, leaving many landlords living elsewhere “completely in the dark” about the new rules.
She added: “One final consideration is how will Rent Smart Wales enforce any fines on those who genuinely didn’t know they had to register. We can see a lot of resistance by those who will claim they didn’t know anything about the scheme.
“Tenants who have any concerns about the conduct of their landlord or agent can report this via the Rent Smart Wales website, however we think an element of common sense should be employed if there are cases of owners located outside of Wales, having not signed up.”