Using Gmail as SMTP for wordpress -> spam

Hello,

I tried to use Google Apps as smtp rather than the native mail function of WordPress.

First, registered users didn’t receive alert mails (I received the mails as failure delivery). On a second time, Gmail blocked WordPress and sent me this warning: “SMTP server error: 5.4.5 Daily sending quota exceeded”…

I don’t understand anything, how can I use the SMTP of my Google Apps account to send mails from my buddypress/wordpress blog ? I read somethings about SPF, and I saw that my DNS had the good SPF :
v=spf1 include:_spf.google.com ~all

I remove it to put the default SPF of my registrar:
v=spf1 ip4:myip/18 ?all

Is it good or do I have to set back v=spf1 include:_spf.google.com ~all ? How can i solve the pb of Gmail and spam ?

Thanks.