Delay reason: SMTP error from re

The error message you encountered—"SMTP error from remote mail server after initial connection: 421 Downstream server error"—indicates a problem with the downstream mail server (i.e., the server receiving the message). This issue is often temporary and caused by the receiving server being overloaded, misconfigured, or temporarily offline.

Common causes and solutions:
Downstream Server Issues: The receiving mail server may be having capacity or configuration problems. This is typically beyond your control, and the issue may resolve on its own as the server returns to normal operation.

Solution: Wait and try sending the email again later.
DNS Configuration Issues: If the DNS settings for the recipient’s domain are incorrect or outdated, the email server may fail to connect properly.

Solution: Verify that DNS records (like MX records) for the recipient’s domain are correctly configured.
Email Server Limits: Some email servers impose limits on the number of simultaneous connections or incoming messages they can handle, leading to this error.

Solution: If you are sending many emails or large attachments, try reducing the volume or size and attempt again.
Server Blacklisting: Your mail server or the downstream server could be blacklisted, causing connection rejections.

Solution: Check if either server is blacklisted and, if so, take appropriate steps to delist.
Security/Firewall Restrictions: Sometimes, firewall or security settings on the receiving end might block incoming connections.

Solution: The recipient's email administrator may need to adjust firewall or security settings.