Question Question about spf configuration

[LaurentR2D2]

Hello
Let’s assume my server hostname is example.com. It is used as mail server for all the domains on my server. I use mail.example.com as imap and smtp server. I’ve put the following spf in all the domains dns :

v=spf1 mx a include:example.com -all

I’ve checked this configuration on several spf websites and the results are good. I still get mails refused because of the spf configuration. What should I do ? Should I change the mx of all the domains dns of my server for mail.example.com ? Or should I do something else ? I’ve also activated DMARC and DKIM.

Thank you