A patron came up to the desk, saying she had an email question.
After a bit of a convoluted story, it boiled down to this: she was applying for a job, and emailed her information to their HR person. But she never got called for an interview, because the HR person said she had never received the patron's information. The patron wanted to know if there was a way to prove that the HR person did get it, because she knows she sent it.
The patron seemed to be fairly knowledgeable about computers and email, but I explained anyway that it is certainly possible for something not to get delivered, or get blocked for whatever reason, or go into a spam folder, etc.
Having a message in her own Sent folder would indicate when it was sent. That can probably be manipulated so I don't know if it'd be admissible in court, but in this case it might be good enough if the HR person was willing to listen.
But what the patron really wanted was confirmation that the HR person received the email. I didn't know how to find out after-the-fact (other than subpoenaing their server logs), so I told her about delivery receipts and read receipts. These are the little confirmation messages that come back to let you know someone got and opened your message.
Since it was the closest thing to what she wanted, we went into her email account so I could show her how to use them. However I explained that these aren't foolproof either - not all email clients will honor them. In fact, the email client I use offers a setting to ignore them.
She had both a Gmail and a Yahoo account, and it turns out - much to my surprise - neither one lets you request receipts.
I did some quick checking online, and it seems like Yahoo doesn't offer receipts at all, and Gmail only with their business accounts (not the free version).
Well, like I said, I was surprised. I tried searching for ways to make it happen anyway, and it looks like there are only two options: use an email client like Thunderbird or Outlook (which, for a patron using a library's public computer, isn't actually an option), or use one of the many email receipt services out there. Another website I found had some trickier solutions, but were too complicated for our purposes. There's also Boomerang for Gmail, but since that needs to be installed in the browser, it likewise wasn't appropriate.
Until this day I didn't even know these existed, so I have no idea how well they work. The patron was interested in the free web-based services, but only future-tense. Unfortunately, it looked like she was out of luck with her original question. I think she knew that before she even asked, but hoped librarians had some magic we could work - I hate disappointing patrons.