Exchange Online generates automatic MailTips to advise email creators that recipients are out of offce or the message is addressed to too many recipients. Custom MailTips for mailboxes, distribution lists, and other mail-enabled objects, including language-specific translations, give additional guidance to users as they create messages. Overall, MailTips are worth spending some time on to get right within an Office 365 tenant.
https://office365itpros.com/2020/08/07/mailtips-useful-guide-good-user-email-habits/
