Newbie Question

I just started work as a tech for an ISP. 
I was setting up email for a client today. 
He changed ISP's because someone 
was sending Spam from his email account(s).
I understand I can set up an SPF record on his end.
How do you get all other mail servers to check for the record? 
It seems that even if this is set up, and millions 
of pieces of spam are being blasted as coming from this
account, unless ever mail server receiving them is 
checking for SPF records,
it's not going to help. Am I reading this wrong? 
Any guidance would be appreciated.

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