Are one time passwords better than one time links for deliverability?

Key findings

• MBP tracking: Mailbox providers (MBPs) do not rely on image downloads or even explicit clicks in the way senders often assume to track engagement. They have internal, server-side analytics and other signals (e.g., IMAP flags, user interactions within their webmail interfaces or apps) to determine if an email is opened or acted upon.
• Deliverability parity: In terms of core email deliverability, there is virtually no difference between using one-time passwords and one-time links. Both are generally considered equally successful at delivering the necessary verification token. Neil White explored this directly, stating that both methods are equally successful in delivering the token required by the system. Find out more about if email providers track clicks on verification links.
• Sender analytics vs. MBP metrics: Senders track opens and clicks primarily because this is the data they can easily access. Mailbox providers, however, use a much more comprehensive range of engagement metrics, which are not transparent to senders. Understanding your overall email domain reputation is far more critical than focusing on individual click rates for deliverability.
• Security implications: While less directly related to deliverability, one-time links can carry security risks if prematurely clicked by automated systems (e.g., link scanners, crawlers) or cached, potentially rendering them useless before the intended user can access them. One-time passwords are generally seen as slightly more secure in this regard.

Key considerations

• User experience: Choose the method that provides the best and most secure user experience. OTPs can be quicker as they avoid a click-through, but OTLs can be more convenient as they remove the need to copy and paste. Consider which method aligns better with your application's flow and user base.
• Internal analytics focus: While clicks aren't a primary MBP deliverability signal, they remain a crucial metric for your internal analytics. They help you understand user behavior, conversion rates, and the effectiveness of your calls to action.
• Deliverability fundamentals: Focus on the core pillars of email deliverability: strong sender reputation, proper authentication (SPF, DKIM, DMARC), high-quality list hygiene, and sending relevant content. These factors far outweigh the choice between OTPs and OTLs.
• Mitigating link risks: If using one-time links, implement robust security measures to prevent pre-emptive clicks or caching, such as single-use tokens, short expiration times, and IP-based session validation. This ensures the link's integrity until the legitimate user accesses it.

Key opinions

• Click preference: Many marketers believe that a user actively clicking a link in an email is a stronger engagement signal for sender reputation than merely opening an email, especially for transactional or authentication messages.
• Image download misconceptions: Some marketers mistakenly believe that the non-download of images in a one-time password email (where the password is visible without image loading) signifies a lack of engagement to mailbox providers, unlike a one-time link which necessitates a click-through to a web page.
• User experience focus: Marketers also consider the user experience of OTPs and OTLs. OTPs can offer convenience by not requiring a browser visit, while OTLs provide a direct path to the intended action, potentially simplifying the user journey. The expected delivery times for OTP emails are also a key consideration for user experience.
• Internal data limitations: Marketers often rely on the open and click data provided by their ESPs as the primary, and sometimes only, available engagement metrics. This can lead to a belief that these are the same metrics MBPs use for filtering.
• Enhanced security claims: Marketers often highlight the security benefits of OTPs, such as their resistance to replay attacks and the fact that they are valid for only a single use, which enhances overall account security and user trust, as MailerSend notes in their discussion on OTP messages.

Key considerations

• Re-evaluate MBP tracking: Marketers should understand that MBP engagement tracking is sophisticated and goes beyond simple image loads or raw clicks. Focusing too heavily on these specific metrics for deliverability may not yield the expected results.
• Holistic deliverability view: Shift focus to a broader deliverability strategy that includes strong sender reputation, proper list management, and content relevance, rather than overemphasizing the perceived deliverability impact of click rates on authentication emails.
• Internal metrics for optimization: While MBP filtering isn't solely based on clicks, clicks remain vital for understanding your audience and optimizing your email campaigns. Use these metrics for improving your own marketing performance, not as a direct proxy for MBP deliverability signals.
• Security implications: When choosing between OTPs and OTLs, prioritize the security best practices and the overall user experience. This choice should be driven by security and usability, not a perceived deliverability advantage.
• General deliverability improvement: Focus on strategies that genuinely impact deliverability, such as maintaining low complaint rates, avoiding spam traps, and ensuring proper email authentication. These factors have a much greater influence on your email deliverability rates.

Key opinions

• No image-based tracking: Experts confirm that mailbox providers do not rely on images to track email opens. Their internal systems track engagement through server-side analytics, such as when an email is displayed in a webmail client or an official app, or when its IMAP flag changes status.
• Click tracking is minimal for MBPs: Many experts state that major mailbox providers do not extensively track clicks on links within emails for deliverability purposes. They have cited privacy concerns as a reason for not performing such invasive tracking. Yahoo provides general guidance for senders, but this does not confirm click tracking for deliverability.
• Sender analytics vs. MBP intelligence: The clicks and opens tracked by senders (via ESPs) are primarily for their own campaign performance analysis and differ significantly from the sophisticated engagement signals MBPs use. Read more about how email providers track verification link clicks.
• One-time link security concerns: Experts caution about the potential for one-time links to be clicked by automated systems (like email security scanners) or cached before the legitimate user can access them, rendering them invalid. This is a security and usability issue, not a direct deliverability one. Understanding how HTTPS/SSL for email links affects deliverability can also be important here.
• Engagement signals are complex: MBPs use a holistic view of engagement, which includes user actions like marking emails as not spam, moving to folders, replying, and general interaction patterns, rather than just single clicks or opens for filtering.

Key considerations

• Focus on MBP-relevant signals: Instead of obsessing over raw click rates for deliverability, focus on broader engagement indicators like low complaint rates, direct replies, and positive interactions within the mailbox provider's environment. These are stronger signals of user satisfaction.
• Security first: Prioritize the security and reliability of your authentication method. If one-time links are chosen, ensure they are robustly implemented to prevent misuse or premature invalidation by automated systems. This is more critical than any perceived deliverability benefit.
• Deliverability fundamentals: Consistent positive sending practices, strong sender reputation, correct email authentication (SPF, DKIM, DMARC), and sending to engaged recipients are the true drivers of inbox placement, regardless of whether you use OTPs or OTLs.
• Transparent communication: Clearly communicate to users how your authentication process works, whether it involves a password or a link, to build trust and improve their experience, reducing potential friction or confusion.

Key findings

• OTP design: Documentation typically describes OTPs as random, time-sensitive, and single-use codes generated for a specific login session or transaction, enhancing security against replay attacks. Descope's documentation defines OTPs as meant for one-time use, making them resistant to data interception and replay.
• Security benefits: Both OTPs and OTLs are considered strong authentication methods because their single-use nature minimizes the risk of credentials being compromised through interception or phishing over time. They are designed to prove user identity for a specific transaction.
• Link security vulnerabilities: While secure, one-time links can be vulnerable if intercepted or clicked by automated systems (e.g., mail scanners, search engine crawlers) before the legitimate user can interact with them. This necessitates careful implementation to ensure validity for the intended recipient only. Consider how link shorteners impact deliverability with click tracking.
• Deliverability not a primary focus: Official documentation on OTPs and OTLs predominantly focuses on their security protocols, technical implementation, and user authentication flow, rather than providing specific guidance on their comparative impact on email deliverability metrics like inbox placement. Understanding how multiple or long links affect email deliverability is more relevant to deliverability.

Key considerations

• Implementation for security: Implement OTPs and OTLs following documented security best practices, including short expiration times, single-use validity, and robust token generation, to mitigate risks such as interception or replay attacks.
• Preventing unintended clicks: If using one-time links, incorporate mechanisms to detect and invalidate clicks from non-human agents or unintended users, such as IP validation or session-based authentication on the landing page.
• Prioritize core email practices: Recognize that the choice between OTPs and OTLs has minimal impact on email deliverability. Instead, focus on fundamental deliverability practices such as maintaining a clean list, authenticating emails correctly (SPF, DKIM, DMARC), and sending relevant, desired content.