SPF has another problem. I sent an email to someone recently @theirdomain.com. I subsequently saw by chance that the email was rejected because they were hosting @theirdomain.com with a random ISP but they had configured the mail to be forwarded to a mailbox in @gmail.com.

Gmail sees the email coming from @theirdomain.com's servers, rather than my server. Gmail checks the SPF record which doesn't match, and it rejects it.

I understand that this style of forwarding is anyway bad because gmail see's all email the user receives @theirdomain.com as coming from those servers, not their true origin. If @theirdomain.com receives (and forwards) any spam, it looks like a spammer to gmail.