One of the reasons I used GSuite was to aggregate all of my emails. I'm in charge of monitoring 18 different addresses.
Some of them aren't real (a combination of catch all and mailbox rules sorts them into a real mailbox), some of them are aliases, and the other ones aren't even with my domain. For example, I want to be able to send email from these three different addresses:
Where I don't have DNS authority over differentdomain.org, I only have a mailbox there. Is there a way to do this? If not I'm going to just switch back to GSuite. As much as I would love to have the features that Office 365 provides, I simply must be able to SEND email from a dozen different accounts as well as receive email, and there's no setting for adding an alias outside a domain over which I have authority.
As Crhis advised you need to use another smtp server.
For example my primary email address is email@example.com but I have as alias firstname.lastname@example.org, in order to use email@example.com to send emails I have to add it as POP3 or IMAP account, link for documentation below, follow Step 3