Ok @lumpkins here is what I would do. You need to be very strategic when dealing with a manipulator.
Make a note for yourself of the persons who have received those emails and when they got them. You cannot use the emails right now but you may be able to use the information in them if you set about getting it the right way.
I would set up a series of 1:1 meetings with the recipients. Say:
- it's very delicate and possibly a HR matter, so you'd appreciate if they kept this discussion confidential as you don't want chatter in the work environment to compromise any investigation
- you've always had a good working relationship with "Jane" and got on well
- recently you've had reason to believe that what she says to you and what she says about you are two different things
- give an example ("she offered to help me with X, unprompted. I was happy to let her go ahead, but I discovered later quite by accident that she was saying to "Dave" that I had told her to do it knowing she was overworked and stressed and this was not part of her responsibilities").
- you suspect this might not be the only time and there are some potential incidents in hindsight where you believe this might have also happened.
- ask if they have any memories or records of complaints about you from Jane, specifically about your conduct in the workplace (you’re not looking for complaints about your perfume or family life choices) either express or implied.
- IMPORTANT: make sure you say they don't have to disclose anything to you right now they're not comfortable with, but if they can recall any incident that would be helpful for HR to have a complete picture.
Keep detailed notes about who you spoke to, when you spoke, if they disclosed anything, if they didn't disclose anything about her but indicated that they were aware of the existence of these complaints (assuming this will be eventually escalated, the investigator can do their own digging and your colleague may feel more comfortable talking to your manager or to HR). You don't need to have done your own complete investigation.
You just need to have enough to go on, and a few witnesses. It's quite possible, if there are a number of others involved, that at least one person will supply you with a copy of an email you already have - and bingo, you have legitimate concrete evidence. I’d start with the most recent witness first, and work backwards.
You've had useful advice but do bear in mind that if you submit a subject access request, a SAR will give you information about yourself but not about another person. So you will not be given the name of the speaker or addressee or context (if it's about another person) as these will be redacted. So an email from Jane to John on DD/MM/YY saying "Doris asked me to help her with this particular task today. It's so unfair, I'm really overworked and stressed right now and it's not my job." becomes a single line saying "email dated DD/MM/YY: Doris asked me to help her with a particular task today."
That's if they even go that far - some companies might take the view that the contents of an individual employee's email inbox are not company records and don't merit necessary scrutiny as these aren’t regularly monitored, so you'll get your HR records and your line manager's opinions but they won't proactively search somewhere else for opinions on you without direction that these exist; and then they cannot tell you who sent or received them.