Given that administrators can’t modify Exchange delivery settings directly when using Exchange Online, something useful to know:
BPOS users can’t send to more than 5,000 distinct e-mail addresses in a single message. This does NOT mean you cannot send the message more than 5,000 distinct recipients (i.e. You can send to ONE Distribution List that has 23,000 members just fine).
Loryan Strant, Microsoft 365 MVP
Consultant, writer and speaker