554 - Message refused by the scanner
Click on “E-mail” to create a new email (or attach to any reply). Select the email you want to forward in the folder, and then drag it to the body area of the email.
General emails: Send us the refused email as an attachment to email@example.com. Or send the email to the above address, as though you were sending it to the original recipient.
421 - Service temporarily not available
SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used in sending and receiving e-mail.
PTR (Pointer records) are used to map a network interface (IP) to a host name. These are primarily used for reverse DNS.
This announcement shows that you have likely reached the maximum limit for parallel SMTP connections. You should also check the PTR settings of your IP address and its matching A records. You can check your PTR setting here.
Connection timed out
You have exceeded the new connections limit. Send further emails via a single TCP/SMTP connection.
Sending Collective Messages Rules