Can O365 account alias addresses send as well as receive emails?

I have used simple Aliases to be able to receive email to Account A and have it show up in Account B. That works and that is what Microsoft told you.

Sending goes from the Default Account. Outlook has always worked this way.

If you wish to send from a different account (not Default), you would have to set up the Outlook Account to do this and the user would have to select the Account they are sending from. This is not automatic.

So you can do what you want but there is a manual element to it.

The best way (and automatable) to do this would be using shared mailboxes.

You can create as many as you want (not require a license and automatically shows up in Outlook) and allow user's Ownership/SendAs -> this way users would be able to send emails from as many email addresses as they want.

Educating them would solve it, but you could also set up forwarding on those shared mailboxes so users would get email to their Inbox.

Might seem complicated but it is a very good way to achieve what you need.

I have found something different. According to this link, OWA should now support sendAs alias: https://www.microsoft.com/en-ie/microsoft-365/roadmap?filters=&searchterms=59437