My understanding of Email Aliases is that you can not use them to send email from Hubspot. To test this, i have set up the additional email addresses as an alias in Hubspot and then attempted to send an email direct from the contact record. The Alias emails are not an option to select sending an email from.
As for the second option, if I understand you correctly, email currently sent to the other aliases is already delivered to the single mailbox. So getting mail into Hubspot isn’t the issue. The issue is making sure that when we send email from hubspot, that we have the ability to switch between the different brands.
To give you a better idea how email is currently configured, please see below:
O365 Mailbox (TRaymond at Acuantia dot com)
----Alias 1 (TRaymond at Tank-Depot dot com)
----Alias 2 (TRaymond at Plastic-Mart dot com)
The mailbox is tied to a user account. All mail sent to any of the three email addresses is delivered to the one single mailbox. Outlook is aware of this relationship and allows the user to change the FROM address on any message sent out from this mailbox to any of the three email addresses (by default, the acuantia.com email address is used unless otherwise specified).
This pattern is used to keep authentication simple for the end user. They don’t have to have three different usernames and password to remember and keep up with. It also allows you to only have a single O365 License in and still be able to send and receive from different domains.