It's almost the end of week already!? Better late than never... a fun challenge!
RegEx FTW. I probably spent more time overthinking how to split the numbers apart from the letters than I should have... as I like to say, "it's always something simple".
It is not very clear, why the example solution has not sorted the mails accordingly and then concatenate all mail addresses. With sorting the mail addresses, the order within each group seems to be different than shown in the example. Has anyone a clue of this?