Remove unnecessary postconf switch '-e' and use single quotes where possible

[casperklein]

Description

This PR streamlines the postconf usage to not use the -e option, as it's the default since Postfix version >=2.8

From the man page: The -e option is no longer needed with Postfix version 2.8 and later.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change that does improve existing functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (README.md or the documentation under docs/)
• If necessary I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes