To align SPF authentication with your sending domain in Google Postmaster Tools, it's crucial to ensure that the domain used for sending emails (5321.MailFrom or Return-Path) matches the domain authorized in your SPF record. If these don't match, there's no authentication connection, and you should ask your email service provider to sign with your domain (or a subdomain). This involves publishing an SPF record in your domain's DNS settings, specifying authorized mail servers and domains. Use Google Postmaster Tools to verify the legitimacy of emails. When DMARC is enabled, SPF aligns with the visible 'From' domain (5322.From). Understand that SPF alignment has strict and relaxed modes. Preventing spoofing is a key goal, and remember to gather authorized sending domains, create the SPF record, add it to your DNS, and test the record. Employ SPF, DKIM, and DMARC for enhanced security.