XenForo Expected response code 250 but got code "503", with message "503 sender already given "

[MilkyMeda]

I haven't come across any source explaining this error. These are just ordinary notification emails. Does anyone have an idea?

• Swift_TransportException: Email to the.last.ksa007@gmail.com failed: Expected response code 250 but got code "503", with message "503 sender already given "
• src/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php:457

Continue reading...