We have been using an internal Microsoft SMTP virtual services server to relay email to Office 365 for some time now with no issues. With the current change with Microsoft only allowing TLS 1.2 it ...

Microsoft has an update for Office 365/Microsoft 365 Exchange Online Basic Auth retirement in the case of SMTP.

I know Office 365 charges per user/mailbox, per month, but does anyone know if there are any limits on the number of secondary SMTP addresses or distribution groups?

Microsoft is working on adding SMTP MTA Strict Transport Security (MTA-STS) support to Exchange Online to ensure Office 365 customers' email communication security and integrity.