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, but had looked at the specific examples of emails the council had deemed unacceptable and considered it disproportionate and unreasonable to have added Feyen and another individual, understood to be councillor Ross Campbell, to the quarantine list without warning.
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."
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," he said.
Feyen said he was still considering his options in the wake of the findings.
"I've received some words from the CE but I don't consider that an apology," he said.
"The ramifications of the findings are huge, of councillors' emails getting messed around with and then only an apology when you're finally cornered. My options are definitely open at the moment."
In a letter to Feyen, Clapperton wrote that he offered his "sincere apologies" for the effect the council's email quarantine practice between 2011 and 2017 had on him.
"The Office of the Ombudsman has made it clear, following its investigation that our practice was unreasonable, and we are genuinely sorry for this," Clapperton wrote.
"Without a clear policy framework or oversight, [the practice] overstepped the mark and consequently had impacted you."
Feyen said he was on his way to a meeting with Local Government New Zealand and would be having a very robust discussion with them.
He said the recommendation the council should apologise was no worse a punishment than "a slap on the wrist with a wet postage stamp."
Feyen believed it was purely an excuse for the chief executive to monitor people he wanted to keep an eye on.
"I'm absolutely happy we've been vindicated to a degree, but the real challenge is to have it looked at properly with a forensic audit," he said.
"I don't know enough of exactly what's happened, but this is big," Feyen said.
"Here we have corporatised governance and the sincerity of the apology is highly questionable. An apology is not enough to be able to carry on in the same way."
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."
"I am confident that the implementation of the new policy will prevent any recurrence of the issues raised by the complainants."
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."
Clapperton said he had quarantined the emails with good intentions "to help protect the health and safety of Council officers and elected members."
A media release from the council emphasised that the Ombudsman found "no basis to suggest Council used its email quarantining process to interfere with or obstruct iwi negotiations, Resource Management Act submission processes, local election processes or Environment Court proceedings."
