Hi @AliceBArcos,
For clarity, you wouldn’t send from a HubSpot domain in the same fashion as you would Gmail. You likely wouldn’t “leave” Gmail to get an email domain from HubSpot. However, from the HubSpot Inbox, you can use a HubSpot Hosted email as your to send email responses and was created when you set up the account.
You can “Use Default” to send from that HubSpot email and have emails sent to your inbox with the Forwarding address. This is typically used if you are sending to an email that is a distribution list vs one that can be connected directly.
Here’s a resource on the conversations Inbox: Set up the conversations inbox
In short, you wouldn’t be able to add a DKIM to authenticate if you are using a free Gmail account, so make sure under the From Address settings you are set to Default instead of “customize from address.”
Hope this helps!