I'm building software that sends login codes and needs inbox placement

To improve inbox placement for login codes, focus on the same fundamentals that mail providers use to judge whether a sender is legitimate and wanted:

1) Authenticate every sending domain

Set up:

• SPF
• DKIM
• DMARC

For best results:

• Use a dedicated sending subdomain for transactional mail, e.g. mail.example.com
• Align the visible From domain with the authenticated domain
• Start DMARC with p=none, then tighten to quarantine/reject once everything is stable

2) Separate transactional mail from marketing

Login codes should be on a separate stream from newsletters/promotions:

• Separate subdomain
• Separate IP pool if volume justifies it
• Separate templates and sending infrastructure

This protects deliverability if marketing mail has issues.

3) Keep sender reputation clean

Mailbox providers heavily weight reputation:

• Send only to users who initiated the login action
• Avoid repeated retries to bad addresses
• Suppress hard bounces immediately
• Watch complaint rates, bounces, and deferrals
• Use consistent sending patterns instead of bursts

4) Make messages clearly transactional

Code emails should look like what they are:

• Clear subject: “Your login code”
• No promotional language
• No image-only content
• Minimal links
• Plain-text or simple HTML
• Include the app/service name prominently

5) Reduce user friction and spam signals

• Don’t send multiple codes unless requested
• Expire codes quickly
• Make sure the email is expected and timely
• Avoid confusing “no-reply” behavior if users need support

6) Maintain recipient trust

Mail providers notice engagement:

• Users should quickly find, open, and use the email
• Encourage users to mark your sender as safe if needed
• Make sure your “From” name is recognizable and consistent

7) Monitor deliverability

Track:

• Delivery rate
• Inbox placement if your ESP provides seed testing
• Bounce rate
• Spam complaint rate
• Domain reputation in Google Postmaster Tools / Microsoft SNDS
• Authentication results in message headers

8) Warm up new domains/IPs carefully

If you’re starting fresh:

• Begin with low volume
• Send to highly engaged recipients first
• Increase volume gradually
• Avoid sudden spikes

9) Avoid common traps

• Shared IPs with poor neighbors
• Broken links or mismatched domains
• URL shorteners
• Excessive tracking/redirects
• Sending to typo domains or inactive addresses repeatedly

10) Use a reputable ESP

If you don’t want to manage deliverability yourself, use a provider that supports:

• Proper authentication
• Dedicated transactional sending
• Reputation monitoring
• Bounce/complaint handling

If you want, I can give you:

1. a technical deliverability checklist for login-code emails, or
2. a reference architecture for sending OTP/login emails at scale.