Sending, receiving, and organizing email in Outlook.com
Does this describe your problem? If so, it is a known bug.
See the Answer by "Hornblower409 May 28, 2026"
https://learn.microsoft.com/en-us/answers/questions/5903667/i-am-not-able-to-send-a-group-email
@jeff m said
I cannot select the list to email everyone on the list
Can you get the List into the "To" field? If so, try clicking the "plus in a circle" next to the list name in the "To" line and expand the list. Any help?
If not, please advise and we'll look for other problems.