At my school we have installed a Squid proxy server to prevent users from logging in to their personal email ids by adding a header into all requests (as outlined here). The proxy itself works fine, and non-school domain email ids are blocked. So, the proxy has been configured as a Global HTTP Proxy on the iPads using the MaaS360 MDM. Everything works fine so far.
Gmail (the app) however does not work. When the app is opened, it loads for a bit and then shows "Something went wrong. Try again later."
Other Google Apps (Drive, Docs, Slides, Sheets) work fine. Sharing files also works, but the email notifying that something has been shared cannot be sent. I can sign in to Gmail Web through the browser though, and there it works fine.
Hence, I am lead to believe that Squid is restricting something to do with SMTP, POP, and IMAP, though that's just a guess. What could be the issue?