Incoming emails and message IDs in headers

[#1]

I have been in communication with a company and notice what seems odd to me. In most messages the ID the ending is associated with the sender e.g. amazon.co.uk but on a few it is not. I have seem IDs ending with @inmx01.plus.net or even a third party.
Can anyone explain in simple terms why the headers may not always apparently relate to the sender?

[#2]

It's dependent on the behaviour of the client used to send the message I believe.

[#3]

Seems odd though that one company should use different email clients, even if the sending email addresses are slightly different,

[#4]

Not really, it's a similar tale at Plusnet. e.g. an automated billing notice, a marketing offer and a direct email from an employee are all sent by different systems.