Need a tool to parse message headers? Trying to break down how long it took to hand off an email message between servers?
Check out this tool from Microsoft, and this tool from Google. Both do basically the same thing -- you paste in the email headers and it will parse them, giving you a breakdown of how much time it took between each server hop.
Here's another Google tool you should bookmark. It lets you decode blobs of Base64-encoded content. Sometimes you'll find this handy when viewing the source of an email message and running into content encoded in this way. I just used it to decode an odd bounce message yesterday.
And here's another thing that a coworker shared with me -- Mail Tester helps you check your emails against SpamAssassin in an easy-to-use way. Check it out!
April 20, 2018 Update: Here's another neat tester: This widget from Litmus tells you what Gmail tab a message gets delivered into.