I'm looking to set up a webmail server that will be used by a lots of users that will receive and send emails. They will also have the possibility to forward emails they receive.

I'd like to know which steps are recommanded/required to indicate to others Mail services (GMail, Outlook, etc) that my server is not used as a spam sender (disclaimer : IT's NOT ! :p) but a legitimate one.

I know I have to define a SPF TXT records for example, but what others steps would you recommend me to do ?

For example, is there a formula like having a proportional number of servers based on the amount of email sent (for having a different IP address) ? (something like sending a maximum of 1M emails / per IP / per day ?)

Something else I'm missing ?

I tried to search online, but I mostly find how to avoid emails sent with scripts (like PHP) being put in the SPAM folder.

I'm looking for a server/dns configuration side.

Thanks a lot for your help/tips, I appreciate !