The majority of Europe appears to have been affected by the outage, with parts of the US and south Asia also struggling to connect.
Reports of problems with the site began just after 7am on Thursday, as customers took to social media to say they were unable to log-in or read their mail.
Tech website Down Detector said 61 per cent of people reported problems with the Yahoo website, while 38 per cent had struggled to log-in.
Down Detector said it had logged over 5,000 reports of Yahoo Mail outages, but the actual number of users affected is expected to be considerably higher.
Although the reason for the issues remains unclear, Yahoo posted an apology on its Twitter account in which the company said it is “working extremely hard” to fix it.
"You may not be able to access some of our services, including email,” Yahoo tweeted, “Our top priority right now is getting this fixed. We appreciate your patience.”
“We understand how difficult this must be for you. We are working especially hard to fix this. Please stay tuned for updates.”
Some users reported being unable to log in for hours on Thursday morning as thousands took to Twitter to complain.
Yahoo Mail has more than 200 million active users and is involved in the transmission of roughly 26 billion emails every day.
The company has been plagued by controversy in recent years after approximately 3 billion users had their information compromised in 2017.
Users of AOL Mail, which is owned by Yahoo’s parent company Verizon, also reported being unable to access their accounts.
An AOL spokesperson wrote on Twitter: “We apologize for the inconvenience you are experiencing.
“At this time, we have identified the issue and it is being addressed. While we cannot provide a specific time when the issue will be resolved, our team is working to correct it as soon as possible.”