Hi Gerald J, I understand how annoying this can be you expect your emails to look clean and professional but instead you’re seeing that strange X.500 address in the header which feels confusing and unnecessary.
This happens when Outlook uses an old Exchange ID instead of your email address usually because of cached contacts after a migration. To fix it clear your autocomplete cache delete outdated contacts pick your correct email when replying and if possible add the old ID as an X.500 proxy. If you’re on IMAP switch to Exchange for better compatibility.
When you check the full message headers does the X.500 address appear only in the From field internally or do you also see it in the Reply - To or SMTP headers?
Best regards
Lucky Charm