One subject, one thread please.
Regarding your postfix listener, best would be to post it there.
Regarding the APT dependency problem this is not due to i-MSCP. I think you've done partial upgrade. Note that the PPA repository is added by the i-MSCP installer (expected). This has nothing to do with the PhpSwitcher plugin but yes, in current version of the PhpSwitcher plugin, there is dependencies problems.