Exchange Mail Flow Guides

These guides answer the questions Exchange administrators ask about getting mail into and out of an on-premises Exchange Server: POP3/IMAP retrieval, send connectors and smart hosts, sender addresses in Outlook, and OAuth2 for Microsoft 365 and Gmail.

Each guide opens with the direct answer, cites Microsoft Learn wherever we rely on Microsoft’s documented behaviour, and points to a product only where a product is part of the answer. Guides are added one at a time. Until a guide exists for a question, the lists below point to the pages that already cover it.

Updated on 2026-09-10

Inbound: POP3 and IMAP retrieval into Exchange

Getting mail that arrives in provider mailboxes (IONOS, Strato, GMX, Gmail, Microsoft 365) into on-premises Exchange mailboxes, and the questions that follow once it works.

• Exchange POP3 connector — what a POP3/IMAP connector does and which Exchange versions it serves.
• Configure Exchange 2013 / 2016 to receive mail from POPcon — the receive-connector setup; guides for Exchange 2010, 2007 and 2003 sit beside it.
• Which Exchange versions and SBS editions does POPcon work with?
• Why does a connector need to know the accepted domains? — the catch-all mailbox question.
• Mailing-list mail is not distributed correctly and BCC recipients do not receive the email — the two classic catch-all follow-ups.
• Only messages under 10 MB come through and how to raise the Exchange size limit.
• Non-delivery reports for unknown users behind a POP3 connector.
• Could not reach Exchange SMTP server — port 25 blocked.

Outbound: send connectors and smart hosts

Routing outgoing mail from Exchange through provider relays, including several smart hosts and authentication per sender.

• Multiple send connectors for Exchange — routing by sender address to different smart hosts.
• Creating the send connector in Exchange manually.
• 504 5.7.4 Unrecognized authentication type — when relaying through the Office 365 SMTP servers fails.
• IONOS blocks non-delivery reports and out-of-office messages — the empty-sender rule at the IONOS relay.

Sender address in Outlook

Letting Outlook users choose the From address per message on Exchange on-premises, and keeping that choice under control.

• Change the sender address in Outlook — per-message sender selection for Exchange users.
• Using the same sender address for two Exchange mailboxes.
• Restricting which Send-As addresses users may choose.

Authentication and SMTP errors

Modern authentication for the mailboxes a connector reads, and the SMTP responses Exchange returns when a connector submits mail.

• OAuth2 for Microsoft 365 and Outlook.com mailboxes — keep fetching by POP3/IMAP after Basic authentication.
• Gmail with modern authentication (OAuth2).
• 530 5.7.0 Must issue a STARTTLS command first.
• 530 SMTP authentication is required.
• 550 5.7.1 Unable to relay.
• 421 4.4.2 Message submission rate exceeded.
• 421 4.4.1 Connection timed out.
• 452 4.3.1 Insufficient system resources.
• 451 4.7.0 Temporary server error (PRX).

Everything else is in the knowledge base. If your question is not covered, ask support — unanswered questions are how the next guide gets chosen.