The company said the new ownership will retain all existing FastMail-related staff, and will be looking to roll out new features over the coming months.
The blog post says that Opera's decision to sell FastMail was not related to any financial issues, and that the decision was reached on a mutual basis.
"Opera has undergone an internal change of strategic direction, and an email service no longer fits within their long-term vision.
"With Opera's investment in development and infrastructure over the last three years, FastMail has continued to increase its rate of growth and profitability."
In an online landscape dominated by free email providers, FastMail has been able to provide an ad-free, subscription-based email service since 1999. Although the company is based in Melbourne, its servers are housed in New York.
Large email providers have been increasingly regarded with mistrust and scepticism following revelations of the NSA's data collecting operations. In the case of Microsoft, the company even helped the NSA break its own encryption scheme.
Both Google and Microsoft have sued the NSA in an attempt to bring a level of transparency to the NSA's actions — American telco giant Verizon has dismissed the suit as "grandstanding" and "fizzy statements".
Last month, secure email provider Lavabit shut itself down rather than comply with a suspected court order.
During the past few months, Opera has discarded its Presto browser engine in favour of a new rendering engine called Blink that is made by Google for its Chrome browser. The company earlier this month also released Coast, a browser for the iPad that removes much of the traditional browser interface and replaces it with gestures.