I've been running Mailinabox since late 2016 and have experienced 0 issues with downtime or my emails being flagged as spam. It did take a few weeks to train SpamAssassin to properly flag incoming email as spam but that worked very well and it. just. works. nowadays.

Recently I closed down my lingering Gmail account and used imapsync to copy over all my email and contacts to my Mailinabox.

I've also moved it to a different VPS provider once without any issues. Setup mailinabox on new server, close ports on old server, create final back-up, import back-up, switch DNS, done. 0 emails missed.