SMTPSendFailedException or how to deal with overload

Question

Java

Actually I am writing a service for sending messages to the mail. I am writing using Spring Mail. Since the mailing can be very large, I run it in several threads at once. However, sometimes not all messages are sent, motivated

com.sun.mail.smtp.SMTPSendFailedException: 421 4.7.0 Temporary System Problem. Try again later (WS)

As far as I understand, this is due to too high a speed of circulation. If you synchronize JavaMailSenderImpl, this error never occurs, but the message transfer rate drops very much. If anyone has experienced this problem, please advise how to get around the problem.

Answer 1

Many ISPs from unheated IPs (IPs with no reputation, senderscore.org for example) do not accept more than 100 messages per hour.
You do not need to run in threads, but more IPs and domains and a message queue.
If you have sufficient financial backing to buy a PowerMTA, do so.