I have checked the eMail header of the eMail from Sender X in the Exchange mailbox of User A and I see Sender X is using Mozilla Thunderbird and I tested it with Thunderbird also, but it works for me.
I cannot provide all details of the eMail from Sender X because it's a customer of another customer, but I have replaced the IP addresses and other private information in the eMail header and this is the result: