Boshier's investigation focused on a period from 2011, when this practice was first introduced, until August 2017 when it was suspended.
He found there was no formal policy in place over this six-year period, meaning the practice went largely unchecked.
"The reasons why some of the particular individuals appeared on the list weren't documented. Three of the complainants were never informed that their emails were being quarantined."
Boshier said the quarantine list included elected representatives as well as some constituents.
"Some emails from those on the list to council staff and councillors were blocked.
"In principle, I think constituents should be able to communicate with their elected representatives. This is fundamental to democracy."
The council began quarantining Feyen's emails in 2015, during his first term as a councillor.
According to the Ombudsman's report, the blocking was imposed after Feyen sent an email to all staff. The council then stopped blocking his emails in August 2016 prior to Feyen being elected mayor.
Boshier said he realised the chief executive was obliged to put reasonable measures in place to protect staff from abusive or offensive messages.
"However, I have looked at the specific examples of 'unacceptable' email correspondence and I consider the decision to add Mr Feyen and another individual to the quarantine list without warning was disproportionate and unreasonable."
Boshier said a high number of the quarantined emails from the five complainants never reached their intended recipents.
"Overall, I believe the council took a cavalier approach to forwarding quarantined emails to their intended recipients, including elected representatives."
Boshier noted the council had since developed a new policy and sought advice from his office.
He said the policy - introduced in October last year - has addressed his administrative concerns.
"The new policy defines what unacceptable behaviour will trigger the quarantine process.
"Affected individuals are notified, have the right to complain and their status is reviewed every six months. The policy also excludes emails sent to elected officials."
Horowhenua District Council Chief Executive David Clapperton said the Council accepted the Ombudsman's findings.
"We fully accept this opinion, and sincerely regret the impact our actions had on the five complainants."
"I am confident that the implementation of the new policy will prevent any recurrence of the issues raised by the complainants."
"The practice was introduced in 2011 and continued by me with good intentions – to help protect the health and safety of Council officers and elected members."
Boshier said there were lessons to be learnt from this case.
"We already have comprehensive guides on managing unreasonable complainant conduct on our website. My office is happy to work with local authorities on providing more guidance and training on this issue."
