Ok, I do NOT use Horde on Slackware but Horde comes built in on SME Server.
Now, you may not be familiar with SME Server, so let me explain.
I am a DIEHARD Slackware user, DIEHARD. But I love to sleep, so when it comes to mailservers, I install SME Server. There's a story behind this.
One day, I get a call from my youngest brother. He's an ace programmer, the best I know. But up to this point, he's pure Windows. So, he says, "I need a mail server and I need it NOW! What can you suggest?".
I say, "Go to https://wiki.contribs.org/Main_Page
, download the iso and call me when you're ready to install it". We hang up.
Six months later, he calls again. "I'm at a customer's site. I've installed SME Server but I've forgotten how to enable email so it works from the internet".
I'm like "WTF are you talking about?"
He: "Remember I called you about a mail server?"
Eventually, I remember the conversation. "But you never called me back, so I thought you didn't use it."
He: "Well, it was so simple to install that I never bothered to call you back. But how do I enable email?"
Me: "Go to the menu option and click on enable internet access to email, duh."
So, he set up SME Server despite never having installed anything Linux before and he was confident enough to install it on a customer site. Need less to say, he is now a Linux user, hardcore. He likes to sleep at night too.
This is the kind of mail server you want. The set-it-up-and-forget-it kind. Trust me.
So, if you need Horde, SME Server is the one. Also, in the contribs section of the site, other components of Horde are available as well.
I've been using SME Server for over 10 years now. It is ROCK SOLID and dead simple. This is really an amazingly capable distro that doesn't get enough love.
Save yourself the headache. Oh, and it runs in a VM as well if you really must run Slackware.