I'll leave the others to address most iof this but as far as different configs goes take a look at the -Y cmdline option, which allow you to have multiple "Pmail.ini" files (by other names of cause) and call them from your shortcut

Easiest way would be make a copy of PMail.ini rename it for your other OS's (W7Pmail.ini, W10Pmail.ini and "maybe" NixPmail.ini) add -Y to your cmdline then make the OS specific changes

If you get the single Mail dir to work under Linux I would love to know the details as I have been trying that on a dual boot (Xp Pro, Ubuntu Xenial, PuppyXenial, Ubuntu Ardvark) netbook without much success (so far)

Is it possible, and if yes how, to install PM in such way that the program (incl. settings) is stored completely separated from the e-mails (incl. new e-mails), e.g. all program files on disk1 and all e-mail files on disk2.

Purposes:

1. easy back-up e-mails (disk2);
2. easy second installation of PM (with other settings than on disk1) on disk3 with access to the e-mails on disk2.

You can put the mailbox directory(ies) wherever you want by specifying that path during install.  Some things to keep in mind:

• Multiple instances of Pegasus Mail can not access a mailbox simultaneously.

• There is little you can do to have different settings in a different Pegasus Mail installations if they're accessing the same mailbox because configuration files are stored in the mailbox directory.
  

Let us know exactly what you are trying to accomplish by your desire to have different settings in the second installation so we can offer advise specific to that desire.

[quote user="Brian Fluet"]
Multiple instances of Pegasus Mail can not access a mailbox simultaneously.  [/quote]

 It is not about simultanously, but alternately (from different operating systems).

[quote user="Brian Fluet"]

There is little you can do to have different settings in a different Pegasus Mail installations if they're accessing the same mailbox because configuration files are stored in the mailbox directory. [/quote]

Little, absolutely nothing? And is it impossible to separate the config. files from the e-mail files?

[quote user="Brian Fluet"]

Let us know exactly what you are trying to accomplish by your desire to have different settings in the second installation so we can offer advise specific to that desire.  [/quote]

To accomplish a multiboot system, e.g. with Windows XP, W10 and (if possible) Linux.
In each OS one PM-installation, possibly different versions of PM.
Each PM-instance works with the same e-mails.

Pasco.

Program files are separate.  Configuration files are specific to the Pegasus Mail user (even if there is only one).  Mailbox directories are specific to the Pegasus Mail user.  Logically, configuration files for each user reside in the that users mailbox directory.

If you are accessing mail as the same Pegasus Mail user from each machine or OS then you essentially have a network installation.  This is when one mailbox exists per user, somewhere on the LAN, but they can access it from any machine on the LAN.  It will always look and behave the same (with one caveat noted below).  There is also the option to have a single, shared Pegasus Mail install on the LAN that can be run from any machine.  Upgrading a single installation is much easier than upgrading one on every machine. 

Any configuration change will affect all instances that access the same mailbox.  The only way to get around this is to create an individual identity for each instance.  Most configuration settings are identity specific allowing you some flexibility in the configuration but the files still reside in the same mailbox directory.

The one caveat to the look and behavior occurs if you are running OS's at different screen resolutions.  The window sizes/positions in Pegasus Mail will be affected.