I'm using Fastmail for my emails.
I just need to know how to check where emails that come in from people using outlook, gmail, hotmail etc into my Fastmail account.
I do not understand what a Full Header is or how to get into the information within the full header
The full email headers show information including the mail server names and IP addresses that an email traveled through between the sender and recipient.
Not all email headers can be trusted.
Generally only the email headers created by mail servers that you trust can be trusted.
The full email headers do not always contain the IP address of the person who sent an email because not all email services include that information when the email is sent.