I'm wondering if I have to include the SPF of any ISP used by my customers to send email with an email hosted on my server.
Example : they use Bell to send email for firstname.lastname@example.org (where example.com is hosted on my server).
Basically, because port 25 is disabled by their ISP, they use their SMTP to send emails. This means that Gmail, Hotmail, etc. see an email sent by Bell (example) but the domain is on my server. The result is a softfail because I end my SPF record with ~all.
Sometimes, it's a local Exchange that handles all email deliverability and they can't use my SMTP.
In a long/short term, can this be a problem for my reputation?
My server doesn't accept clear text authentication, neither authentication through port 25. Only SSL/TLS is allowed.