Gmail Headers
Open mail.google.com
Open the email you wish to view the headers of.
On the top right of the email next to reply, click the three dots (more) -> Show original.
Source: https://support.google.com/mail/answer/29436
Outlook Web
Open outlook.live.com
Open the email you wish to view the headers of.
On the top right of the email, click the three dots -> View -> View message source.
Outlook App
Open Outlook.
Open the email you wish to view the headers of.
Click File -> Properties.
Headers will display in the "Internet headers" box.
Apple Mail
Open Apple Mail.
Open the email you wish to view the headers of.
Click View -> Message -> All Headers.
Headers will display in the window below your inbox.
Source: https://support.apple.com/guide/mail/show-detailed-email-headers-mlhlp1089/mac
Don't see your email client listed? Odds are that a quick search of your mail client's documentation will show you how to view email headers. If you're still having issues, give us a shout and we'll help you find them!