What causes 'Server is not currently available, possibly due to too many connections' and how to fix it?

Email marketer from Email Marketing Forum advises implementing a robust throttling mechanism to avoid overwhelming recipient servers. Gradually increase sending volume and monitor for errors.

Marketer from Email Geeks mentions he has passed the info on to the team to see if there is any more info that can be obtained about the issue(s)

Email marketer from StackExchange explains that the error could stem from exceeding a connection limit or encountering a temporary failure on the receiving server. He suggests checking server logs and monitoring for patterns to diagnose the underlying cause.

Marketer from Email Geeks confirms that the Proofpoint issue should be resolved.

Email marketer from IONOS explains that the error indicates the recipient server is currently unavailable, which might be due to overload, maintenance, or other issues. Solutions include waiting and retrying later, checking the recipient server's status, and ensuring your sending server is properly configured.

Marketer from Email Geeks suggests trying 1 parallel connection, reducing sending volume and frequency, and allowing the system time to recognize the changes.

Marketer from Email Geeks explains it's a basic hourly/daily limitation and suggests waiting and resending.

Email marketer from Pepipost suggests analyzing server logs to identify patterns in deferrals and adjusting sending practices. Also, confirm your DNS records are configured correctly.

Email marketer from Email Admins Forum recommends contacting the recipient's mail server administrator to request an increase in your sending rate limits if appropriate.

Email marketer from Mailjet indicates you should attempt to reduce the number of connections by limiting the number of emails you send at a time.

Email marketer from Reddit suggests the recipient server is likely overloaded and cannot accept more connections. You may need to throttle sending and retry later.

Email marketer from Rackspace advises that this error message usually means the mail server has reached its connection limit. They suggest pausing sending, waiting for a while, and then retrying to send the emails.

Email marketer from SendGrid explains that this error typically means you are exceeding connection limits set by the receiving mail server. Lowering your concurrency and increasing the time between deliveries will help. Consider also checking your IPs reputation.

Expert from Word to the Wise explains that exceeding connection limits can lead to temporary rejections. She suggests ensuring your sending infrastructure is configured to respect recipient server limitations and to monitor sending volume closely.

Documentation from Exim Wiki describes how Exim mail servers use rate limiting to prevent abuse. It suggests configuring appropriate connection and message rate limits.

Documentation from RFC Editor defines the 421 SMTP error as 'Service not available, closing transmission channel'. It indicates a temporary issue, and the client should try again later. The server might provide additional information.

Documentation from Microsoft Support explains that Exchange server might temporarily reject connections to protect its resources. The solution involves adjusting the throttling policies on the Exchange server.

Documentation from Postfix.org explains that the error indicates the Postfix server is unable to process the request due to connection limits, and suggests checking the server's settings.