Where are out-of-office (OOO) messages typically delivered?

Key findings

• Inconsistent delivery: OOO messages may be delivered to the From, Reply-To, or Return-Path addresses, with no single prevailing norm.
• Client-level implementation: The delivery behavior of OOO messages is largely determined by how individual email clients or server systems implement them, rather than a universal standard. Microsoft Outlook, for example, has specific settings for automatic replies.
• RFC recommendations: While RFCs may suggest the use of Return-Path, this is not always followed in practice and is sometimes considered the least desirable option for human-readable replies. Super User discussions highlight this discrepancy.
• Legacy behavior: The varied behavior is often a legacy from earlier email days when the From and Return-Path addresses were commonly assumed to be the same.

Key considerations

• Lack of control: Senders have little to no control over where an OOO message will ultimately be delivered or what format it will take.
• Impact on automated systems: For transactional emails or automated systems, OOO messages arriving at the Return-Path (envelope sender) can mean the original human sender never sees the reply. This is relevant when considering how companies handle out-of-office replies.
• Bounce processing: Many email systems include OOO messages in their bounce processing to account for instances where they are sent to the Return-Path, even if this is not ideal for user experience. Understanding unformatted out-of-band bounces can be helpful.
• Human-intended communication: OOO messages are primarily designed for human-to-human interaction where the Envelope From and Header From are typically the same, making the precise delivery header less critical for the end-user.

Key opinions

• Unpredictable replies: Marketers frequently express frustration over the inconsistent behavior of OOO messages, noting they can appear in the From, Reply-To, or Return-Path fields.
• Impact on metrics: There's a concern that OOO replies, especially if miscategorized, can skew bounce rates or engagement metrics, making it harder to gauge campaign performance.
• Handling auto-replies: Marketers often seek ways to properly handle OOO replies from transactional emails, as these messages can cause issues for automated sending systems.
• NoReply address considerations: A common point of discussion is whether OOO replies to noreply addresses hurt deliverability.

Key considerations

• Processing OOO messages: Marketers should have a strategy for processing OOO messages, treating them differently from hard bounces or complaints to avoid unintended list cleaning or sender reputation damage.
• Transactional vs. marketing emails: The impact of OOO replies differs between transactional and marketing emails. For transactional sends, receiving OOO can be useful for internal tracking, while for bulk marketing, it might just add noise.
• Automated response management: It's crucial for email platforms to distinguish OOO messages from other types of bounces (like a 550 internal out-of-band auto-reply vacation mail bounce) to ensure proper email deliverability management.
• Setting expectations: Clear OOO messages can also be a tool for marketers to set expectations with their audience, as discussed by Michelle Gielan on Harvard Business Review.

Key opinions

• No universal standard: There is no single, widely adhered-to standard for where OOO messages are delivered, with different clients making different choices.
• Client-side implementation: The delivery mechanism is determined by individual email clients and servers rather than a unified protocol. This client-level control often makes OOO messages reply to the From address or Reply-To.
• RFC discrepancies: Although RFCs might recommend Return-Path, experts view this as potentially the worst choice in practice for human-intended replies.
• Control and format limitations: Senders have no control over where OOO replies are sent or what format they will use upon receipt.

Key considerations

• Legacy assumptions: The current varied behavior is rooted in a legacy assumption that the From and Return-Path addresses would always be identical, which is often not the case for automated emails. This can lead to unformatted late bounce error messages.
• Human vs. automated emails: OOO messages are primarily designed for human-originated email where the envelope and header From are typically the same, making the delivery point less problematic for human senders.
• Ongoing discussions: There are ongoing industry discussions about how to standardize OOO message delivery, but a widespread solution is not anticipated soon. This is a common theme when considering broader email deliverability issues, as highlighted by The TechSmith blog's advice.
• Bounce management: It remains important for email platforms to correctly identify and process OOO messages to avoid misinterpreting them as hard bounces, especially for handling email bounce messages efficiently.

Key findings

• RFC 5322: This RFC defines the format of Internet Message Format, including fields like From and Reply-To. Its specifications guide, but don't always dictate, actual OOO behavior.
• RFC 5321 (SMTP): This RFC details the Simple Mail Transfer Protocol and the use of the MAIL FROM (envelope sender or Return-Path) address, which is typically where automated system messages like bounces are sent.
• Auto-reply mechanisms: Documentation for email services like Zoho Mail generally indicates that out-of-office messages are sent in response to all incoming emails, regardless of direct addressing. Zoho Mail's help page confirms this behavior.
• Outlook settings: Microsoft Outlook documentation outlines how users can set automatic replies, including options to restrict them to internal senders or allow them for external contacts, which directly influences where OOO messages are delivered.

Key considerations

• Header interpretation: Documentation often distinguishes between the various email headers (From, Reply-To, Return-Path) and their intended use for bounces versus human replies, though this is not always strictly followed in real-world systems.
• Standard vs. practice: Despite RFC guidelines, the practical implementation of OOO messages highlights a divergence between email standards and the diverse software ecosystems, leading to inconsistencies. This mirrors what RFC 5322 says versus what actually works.
• System configuration: Documentation for email servers and clients often provides detailed steps for configuring auto-replies, which directly impacts their delivery scope (internal versus external recipients).