There are plenty of mobile applications that you sign into with different accounts to retrieve email, contacts, and calendar (such as CloudMagic), and a couple desktop clients (such as Inky). Yet I can find no web based clients that do so, besides forwarding of messages from provider to provider.
Say I want to create a NodeJS web based email client that has full read and write access (like a normal email client), and it will link to my users' Gmail and Office365 accounts. All IMAP functionality is on the NodeJS backend, but obviously, one of the concerns that this would raise is storing passwords in my own database to access IMAP of the client. I do not want to do this.
But that concern is mitigated to an extent by using OAuth tokens to access IMAP and mail APIs, right? I'm sure I am missing something as I feel like there would be plenty of web clients that exist that do this.