What is solid about exim? Not only does it have more CVEs than any other mail transport agent (including four, all critical or high, just last year), they tend to respond by doing infamous things like releasing security patches on Christmas morning. I'm not the biggest fan of Haraka, but exim is easily the biggest security hassle you can ask for in an email server.
I generally recommend replacing Roundcube with SnappyMail (https://snappymail.eu) -- not having to deal with a database by not maintaining much state is a win.
I was expecting to see Postfix instead of Haraka. I wouldn't have been very surprised at exim.
Why not exim or something similarly solid and well-understood?