An e-mail outage that plagued at least four public agencies on Monday prompted concerns about a cyber-security breach, but this was quickly ruled out.

Instead, a security patch upgrade is believed to be behind the outage, which left several public servants unable to send or receive e-mails for periods ranging from 30 minutes to several hours.

Experts The Straits Times spoke to said such patching issues, caused when a piece of code inserted to improve a program's function is incompatible with other parts, are inevitable.

But it is not a major issue as long as IT staff rectify the problem quickly, and keep the rest of the company informed about the downtime, they said.

While Monday's outage left public servants unable to communicate via e-mail, there are alternatives, such as encrypted messaging apps like WhatsApp or Telegram, to get across the most urgent of messages. More importantly, no confidential data was compromised. Neither were any public services unavailable during the outage.

The e-mail outage was rectified by Monday night, and such issues occur only from "time to time", the Government Technology Agency said.

Digital strategy consultant Ryan Lim said people should be alarmed only if an outage cannot be solved in a day or so, and less concerned about the frequency of such an outage.

"If it is out for a few hours to a day, it shouldn't raise an eyebrow. More than that, and it still can't be resolved? Then there could be 1,001 reasons, including bad IT," he said.

He added that most companies are likely to avoid such outages as they turn to cloud-based servers such as Gmail for e-mail functions, which have redundancy plans to ensure smooth service.

Those who rely on their own servers, including for security reasons, have to contend with a thinner IT staff handling a complex network of servers and various restrictions for staff of different security levels, he added. But e-mail outages are, in general, highly unlikely to arise from loopholes which hackers can exploit later.