Sending mail from other Domains SPF and DNS

Hi

We currently host websites for numerous companies. Each company owns their own domain name and points the www record to our servers.
We now have a requirement to send out mail from these sites as the company in question.

I need to avoid these emails been flagged as spam when we send them out from our mail servers.

if I get each company to add our mail servers into their spf records will this pass any reverse lookups, spam tests that are done on the mail sent out or is there anything else I need to do.

I must not stop each company sending out mail from their own mail servers.

Can anyone advise me on the correct setup

Thanks in advance

Unfortunately there is no straight answer since mail servers all have their own ways of checking for spam. If SPF is implemented by some of your clients, then you would want to add your server info in their record(there are a variety of ways to do so). You shouldn't have much of an issue with SPF.

But when a receiving server does a reverse lookup on the sending IP and sees your server name instead of the sending companies server name, it could be an issue. I've seen that happen but it is bad practice to require such strict settings. So normally as long as you don't have a generic PTR record on the sending server, mail will be accepted.