I'm using postfix to send some mail from my cloud server via a web page on my website that uses PHPmailer to do SMTP authentication. My cloud server has a dedicated IP for all inbound and outbound data. The problem I am having is that it sends roughly 75 - 100 messages successfully before it returns the following error for the remaining few hundred recipients in my list:
SMTP Error: The following recipients failed: firstname.lastname@example.org
Mailer Error (email@example.com) SMTP Error: The following recipients failed: firstname.lastname@example.org
SMTP server error: 5.5.0 Sender already specified
Anybody ever come across this before? What on earth is happening here?