I am not an IT professional, but am in need of help from one...or two.

To make a long story short, I was having a discrepancy with a previous employer about the existence of an email. They claimed that they could not find it, and unfortunately, I had canceled my account with my ISP and no longer had the original email. I contacted the ISP (Verizon) and they were able to provide me with the email and full headers.

Prior to this, I knew very little about headers. When I sent the company the information from Verizon, they told me that they still questioned the message because they were still unable to locate it on their server. I've never had much confidence in their IT team. They are saying that they were using the "Message-ID" to located it and were unable to do so. I've tried to research what the purpose of the message-id is, but other than knowing that it can contain a time-date stamp (which this one does, plus another little code after it) I don't know much else.

My questions: Is a message-id a piece of an email header that an IT professional could use to find the email on the server? Are there any other components of a header that could be used? Are they just messing with me?