This is bad, and I'm not letting Microsoft off the hook here. But, there's an easy workaround for this particular case: Use an alias, such as myrealemail+msft@example.com, for the new account.
This is why I use Postfix and Cyrus for my personal mail. I get Postfix to pre-parse transform the '-' char to '+'; this way I can use email addresses that stupid systems don't barf at but can still get mail delivered to the right mailbox (including sub mailbox if it exists). e.g. me-junk@mydomain.tld goes directly to my junk mail box :)
I should have mentioned in the previous post, recipient_canonical_maps is probably what you are looking for. Depending, trivial_rewrite might also fit your use case depending on how early into the delivery process you want the addresses to be rewritten.