Proofpoint

smtp;421 4.1.1 <email@example.com>: Recipient address rejected: unverified address: host example.com[x.xx.xx.xx] said: 450 4.1.1 <email@example.com>: Recipient address rejected: unverified address: lost connection with mail.example.com[x.xx.xx.xx] while sending RCPT TO (in reply to RCPT TO command)

smtp;421 4.1.1 <email@example.com>: Recipient address rejected: unverified address: Address verification in progress

smtp;550 5.1.1 <email@example>: Recipient address rejected: User unknown

smtp;550 5.1.1 <email@example.com>: Recipient address rejected: undeliverable address: host example.com[x.xx.xx.xx] said: 550 5.1.1 <email@example.com>: Recipient address rejected: user unknown

smtp;550 5.1.1 User Unknown

smtp;550 5.1.1 - This is not a valid email address. Please verify that you have the correct email address for your recipient.

smtp;550 5.1.8 <sender@example.com>: Sender address rejected: Domain not found (in reply to RCPT TO command)

High severity

smtp;550 5.1.8 Domain of sender address does not exist

High severity

smtp;550 5.5.2 <email@example.com>: Sender address rejected: need fully-qualified address (in reply to RCPT TO command)

High severity

smtp;550 5.7.0 Email rejected per sender's DMARC policy

High severity

smtp;550 5.7.1 <email@example.com>: Recipient address rejected: User email address is marked as invalid.

smtp;550 5.7.1 Relaying denied

smtp;550 5.7.1 Relay access denied

smtp;550 5.7.1 Service unavailable; client [x.xx.xx.xx] blocked using prs.proofpoint.com

High severity

smtp;550 5.7.1 Service unavailable; client [x.xx.xx.xx] blocked using Cloudmark Sender Intelligence (Visit http://csi.cloudmark.com/reset-request/ if you feel this is in error)

High severity

smtp;554 5.7.1 <email@example>: Relay access denied