M365 (O365), MX etc and new email subdomain for Hubspot and other 3rd-party mail senders

[js_lon]

Hi

My small business has been using M365 (aka O365) for years.  

With the addition of various third party email senders (like xero.com, hubspot.com, gsuite), our SPF record has become too long.  I've decided that Hubspot (marketing) and Xero should send mail from a subdomain instead of the primary one - which apparently is best practice anyway.

e.g. hubspot sends from email.mydomain.com instead of mydomain.com

As a non-IT person (we don't have any!), I'm confused about how to set up MX, SPF etc.

M365 will continue to send/receive mail e.g. from firstname1.lastname1@mydomain.com .

Hubspot might send a marketing email from firstname1.lastname1@email.mydomain.com 

How do I set up MX and SPF records and settings inside M365 so that this all works correctly and I don't break the primary domain? 

...specifically so that if someone replies to firstname1.lastname1@email.mydomain.com the message goes to my mailbox on 365 which is  firstname1.lastname1@mydomain.com .

We manage domains and subdomains at the host (Siteground) through CPanel.  

Feel free to explain this in very basic terms!

Thanks for your help.  

Jonny

[#2]

You should be able to configure a different reply-to address for those emails. Or, you can add the email.domain.com subdomain to your tenant, then configure the MX record to point to O365 and the SPF record as needed. Don't forget to also add user@email.domain.com aliases to any user/mailbox that needs to receive such messages.