421 Service not available

Meaning

The service is not available, closing transmission channel.

Common causes

• Server overload.
• Maintenance.

How to fix

1. Retry later.

Example bounce

421 Service not available, closing transmission channel

Related codes

• 421 4.3.0 Temporary system problem
• 421 4.4.5 Server busy
• 421 4.3.2 System not accepting network messages