Why 2FA codes over email need faster delivery guarantees than you think
A one-time code for 2FA is one of the few transactional email types where users are actively staring at their inbox, refreshing, waiting. A password reset email arriving 90 seconds late is mildly annoying. A 2FA code arriving 90 seconds late, after the user has already given up and hit "resend," is a support ticket and a bad first impression.
What actually determines delivery speed
The API call itself is nearly instant; the variable is everything downstream. Recipient mailbox provider load, whether your sending domain has an established reputation, and whether the message triggers extra scrutiny (a brand-new sending pattern looks different to a spam filter than an established one) all affect how fast a message actually lands in the inbox versus sitting in a queue.
Where email genuinely struggles against SMS for this use case
Being honest about the tradeoff: SMS delivery for OTP codes is typically faster and more predictable than email, since it doesn't pass through the same spam-filtering layers. If your 2FA flow is latency-critical and you're seeing users complain about delay, that's a real signal to at least offer SMS as an alternative, not just a reason to blame the email provider.
What you can actually control
- Keep OTP emails on a well-established sending domain with real history, not a brand-new subdomain nobody's mailbox provider has seen before
- Don't batch OTP sends with anything else, they should be a single, immediate, isolated send
- Set a realistic expectation in your UI ("this can take up to a minute") rather than implying instant delivery you can't fully guarantee
- Monitor actual delivery times through your provider's logs, not just "the API call succeeded", a successful API response only means the message was accepted, not that it landed