Basic Authentication Deprecation in Exchange Online – September 2022 Update

I would direct your frustration towards HubSpot as they should have the resources to figure out another way to interface with 365.  As a tech person I would argue with the idea that we are looking for a way to abandon Microsoft.  In fact this change, while somewhat painful, is entirely warranted.  IMAP/POP3/SMTP with basic Authentication was constantly under brute force password attacks when we ran our own Exchange server and it has continued since we switched to 365, though it's less visible.  Those protocols do not lock out accounts due to multiple failed login attempts, if they did they could then be utilized for Denial of Service attacks.  I would expect Google to follow suit in the future.

We are in a similar situation, we have a Helpdesk Ticketing system that was utilizing POP3/SMTP.  Fortunately our vendor for that heeded the warnings from Microsoft that this was coming for at least the past year.  They implemented a 365 integration that allows that software to read inbound emails and generate tickets.  I do not know exactly how they implemented that but it did require us to register an app in Azure AD that allowed it to work.