At least four government agencies were unable to send e-mails for periods ranging from 30 minutes to several hours on Monday.
These were the Land Transport Authority, the Housing Board, the National Environment Agency and the Islamic Religious Council of Singapore.
Employees told The Straits Times that their e-mails were stuck in the "outbox" and could not be sent. It is unclear how many public servants were affected in total, but no public services were affected.
In a reply to queries, a spokesman for the Government Technology Agency (GovTech), which leads the Government's charge on digital transformation, said "a few users experienced intermittent access to their e-mail", and there was "no widespread outage".
"The issue has been contained and we are confirming the cause," she said. The agency eliminated cyber attacks as a reason.
Its current assessment is that the outage was due to a security patch upgrade. This is when a piece of code is inserted into a software to improve its functioning.
GovTech said such disruptions are "not common" and it puts patches and software upgrades through quality assurance testing before they are sent out. "Still, incidents like this do happen from time to time," said the spokesman.
"At GovTech, we have standard operating procedures and capabilities in place to quickly isolate such incidents and recover the services as soon as possible," she added. She did not say what these procedures were.
Cyber-security experts said that such e-mail outages are inevitable.
"Even the most sophisticated IT vendors cannot guarantee you a 100 per cent no-disruption service. But they can promise you that they will react quickly to solve the problem," said chief executive Glen Francis of IT consultancy CIO Academy Asia.
What is more critical, he said, is that organisations' IT staff are able to react to such outages quickly and keep employees updated on how to handle such disruptions.
Digital management consultant Ryan Lim said the e-mail outage appears to be more severe than it is "as the workforce depends on it as our main means of communication".
But "if it were a cyber attack, you would deny the organisation's access to critical functions like records and data - not its e-mail services", he said.
In a bid to prevent cyber attacks, the Government delinked Internet access from the work computers of all 143,000 public officers earlier this year. They can still surf the Internet, but only on mobile devices or computers that are not connected to the office network.