Is rc.d = init.d?
I need to copy XMail startup script to init.d...not really sure where is it.
Or do I rename it to rc.xmail and use rc.d?

I guess you could look at your folder names like that. Bear in mind that Slack operates differently to RH (assuming thats what you're trying to transfer from), and also RH uses other scripts and functions (checks/error/succes output, etc) to do things. Slackware uses lots of standard checks to do things, so you may have to make a script manually to do what RH does.

Also bear in mind that RH keeps binaries and libraries in different places to Slack/other distros.

So...will it be OK to use rc.d for xmail.sh script? Do I need to do anything to
make it match Slackware way of handling startup scripts?

I don't know. I can't really speculate without seeing it.

Though slack uses BSD style init, it has a basic support for SysV style too. There is a script in /etc/rc.d that provides this (it's called rc.sysvinit, surprisingly )

I've never tried that, so unfortunately I'm unable to tell exactly how it works. Taking a quick look at the script seems to suggest that you simply create the appropriate runlevel dirs under rc.d (i.e. rc3.d for console level, rc4.d for X, rc0.d for shutdown etc.) and place the scripts to be run there with S (start) as the first letter of the filename of the script. Accordingly K (kill) as the first letter would stop whatever needs to be stopped.

Hope this helps