Transient (4xx)
450 4.2.1
Mailbox temporarily unavailable
The recipient's mailbox can't accept mail right now, but the address itself is valid.
Common causes
- Recipient's mail server is under maintenance or temporarily overloaded.
- Recipient's inbox provider is rate-limiting your sending IP.
What to do
- →Retry later - most sending infrastructure automatically retries 4xx codes for a period before giving up.