Emails Are Deleting From My Inbox In One Address When Clearing Sent Folder From Another Address

Hi All, I’ve been experiencing a strange problem. I have several different email addresses (personal, business, etc.). All the accounts are in my Thunderbird program on my desktop computer. Sometimes I want to send an email to myself with links or other info (i.e, a reminder). So I’ll send the email to my Hotmail account from my Gmail account if I’m on my laptop or tablet. This way, the email stays in my Hotmail account on my computer. I intentionally don’t have the Hotmail account on any other device. When I look at my “sent” items in my Gmail account on my desktop and other devices, I see that the sent email is there. When I look at my Hotmail inbox, I see the email is there. If I delete the email from my Gmail “sent”, it also deletes it from my Hotmail inbox. If I move the email from my Hotmail inbox and put it in another folder, it deletes from there as well. I hope this makes sense. This frustrates the heck out of me...any ideas on why this happens or how to correct it?