Office 365 allows mailboxes to be access via POP or IMAP for integration with HelpSpot. We recommend using IMAP in most cases. You can use this documentation from Microsoft to find your IMAP and POP connection settings. The documentation from Microsoft specifies TLS encryption for connecting via IMAP. In our testing, however, SSL no-validate works while TLS does not.
For outgoing mail (SMTP) TLS security must be used to connect and a HELO value of "HELO." You must also ensure that the Reply To: addresses used are addresses that exist in Office 365. Office 365 will reject mail sent with a reply to of a non-exchange email address for security reasons. Below is a screenshot of a sample of working settings.
Office 365 can be slow when sending multiple emails at once. This typically happens when replying to a message that has staff notifications or that match automation (triggers, mail rules) which send additional emails. Office 365 seems to have some throttling in place, but we haven't found a configuration to change that.