Gmail users are not able to use their email because the Gmail IMAP server is down. IMAP is an important service that helps users connect their Gmail account with third-party email clients such as Microsoft Outlook. Without a working server, Gmail does not function through these clients when using IMAP.
The imap.gmail.com site is currently down. According to the first mentions of an issue on Twitter, the server has been down for around two hours, while website monitoring service Down Right Now says Gmail has experienced issues for the past three hours. The Gmail service itself — mail.google.com – is unaffected and working as usual.
The alternative connection — Gmail’s POP server — is working as usual but, since Google actually encourages its users to connect their accounts to mail clients using IMAP, the outage is likely to affect a substantial number of people.