I have had every now and then someone email me but there reply to email address bounces
due to this problem
https://support.office.com/en-ie/article/outlook-com-email-account-appears-as-outlook-long-series-of-letters-and-numbers-outlook-com-in-outlook-for-windows-90ed7938-7f21-4cb5-a69d-a3b79ea4eafd
(thanks to Niko for finding that out)