Common causes of 'message rejected as spam' errors for ESPs revolve around poor sender reputation, stemming from factors like high spam complaint rates, sending to invalid email addresses, lack of proper email authentication (SPF, DKIM, DMARC), and triggering content-based spam filters like Vade Retro. Several blocklists, including Spamhaus (SBL and ZEN), URIBL, Return Path Reputation Blocklist (Validity), and BarracudaCentral, actively identify and block spam sources. Dynamic IP addresses and invalid PTR records can also contribute to deliverability issues. Maintaining good email hygiene, monitoring sender reputation through tools like Google Postmaster Tools, Microsoft SNDS, MXToolbox, and GlockApps, and adhering to bulk sender guidelines from providers like Google and Microsoft are crucial for preventing blocklisting.