The GMail Android app does work with other people's IMAP and POP3 servers, for what it's worth. But then you have to use Android.
Gmail for Android supports third party pop\imap servers quite well.
Outlook supports IMAP, not POP3. https://support.office.com/en-us/article/Set-up-email-on-iPh...
Re:Dropbox closing Carousel and Mailbox
by zimpenfish on Tuesday December 08, 2015 @05:55AM
(#10695723)
Ah, good to know, thanks.
(Well, the Gmail iOS app certainly doesn't; I haven't tried the Outlook client but the description in the app store suggests a very limited set of providers.)