When I & my users receive emails from the server, it is from postmaster@.... instead of noreply@... (as can be seen on the attached screenshot).
This even happens with the test email.

How does this come?

The plugin Mailgun is even uninstalled

It also happend that when i tried to register 2 accounts on 2 different emails, they didn't receive the welcome registration email.

The email adresses where from Gmail and one based on outlook; no emails to be discovered in the spam inbox...

What do i need to adjust? Some mail server adjustments?

Does it mean that some peoples listings don't get processed by the website if the mail server doesn't function properly.
Let me know if this requires a solution ASAP...

To add to this situation; do i need to change the following settings (see screenshot)

I increased the OSC_MEMORY_LIMIT; if I don't then i am not able to access the 'search' tool of the SEO plugin & users aren't able to select their city while registering after selecting country and region.

Check with your hosting, your setup is OK.

@Osclass

Do you recommend turning on the OSC_CACHE?
How come the emails are still send in from Postmaster@...

To be more specific; what should i ask my hosting?

Caching nor memory have anything to do with email ... I'd check with the hosting company if there are any restrictions to your package mailing feature, as MB suggested.

You can setup the sending "from" either from the admin panel or hard coding it in phpmailer under vendor/phpmailer

If it's not properly being recognized and it is correctly set in admin then it's either a problem introduced by maligun (try installing it again, empty/clear your from field in the plugin, save and uninstall it then go back to the mail server in admin and set and save your preferred from there.

Another thing to do is to check your DB to see if the value is being correctly updated there.