As I understand - exim send email from right address - XX.XXX.XX5.187. Then google connected to XX.XXX.XX5.162 for checking a mailbox existing. So, address XX.XXX.XX5.162 is uses for mail delivery.
I think the reason is a mx records of your mail domain. It should point to XX.XXX.XX5.187, but is seems to point to XX.XXX.XX5.162
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