Check the sending result
Confirm the recipient address, visible From address, timestamp, and relay result. An accepted message may still be queued or filtered by downstream mail systems, so acceptance alone does not prove inbox delivery.
Avoid repeated retries while the first copy may be in transit. Duplicates can confuse the recipient and make diagnosis harder. Keep the message identifier and compare it with available delivery logs.
Review authentication and recipient rules
Check that the sending domain authenticates mail and aligns with the visible From address. Ask the recipient to review spam, quarantine, inbox rules, and blocked senders; organization-wide filters may not be visible in their mailbox.
If one recipient is affected, investigate that mailbox and its provider. If several recipients are affected, inspect recent sender configuration changes. Share only sanitized diagnostics and never include credentials or private message content.