I need my phone to play a special, long sound when getting an email from a certain sender or with a certain Subject or Body. If only sender works that is fine too. I tried using Gmail but the sound only comes through very short.

See https://support.google.com/inbox/answer/6067586?co=GENIE.Platform%3DAndroid&oco=1 .

Another App would be OK too as long as it supports IMAP. Zoho Mail for example would be fine.

I was also looking at http://forum.flymeos.com/thread-24183-1-1.html but could not see the "Unimportant Notifications  Manage" option.