Gmail does it to external mail servers as one of many mitigation strategies. I mean, when you send batches of mails to gmail, it lets them through first, then

Gmail does it to external mail servers as one of many mitigation strategies. I mean, when you send batches of mails to gmail, it lets them through first, then depending on its whim it starts throttling or outright blocking them. Once in a while, it changes its handling of DKIM or SPF or god knows what and the only thing you can expect from them is an SMTP error message.