Customer.io

Customer.io can deliver your messages through an SMTP server you provide. Menu names change between plans and versions; look for the setting that lets you use your own SMTP provider or add an SMTP email service. Reference: Send with SMTP.

#Before you start

1. Verify the sending domain in usend.
2. Create a sending_access API key limited to that domain.
3. Check that your usend plan covers your volume: see Plans and limits.

#Enter the connection

Send a test message from Customer.io to an address of your own and confirm it appears in the usend dashboard under Emails.

#What to know before you move campaigns

• Rate: usend accepts 10 messages per second per API key over SMTP. For bigger bursts Customer.io will see temporary 421 replies and retry; it will take longer to drain.
• Unsubscribes: Customer.io handles its own unsubscribe links. usend passes List-Unsubscribe and List-Unsubscribe-Post headers through (what SMTP keeps), so one-click unsubscribe keeps working.
• Bounces and complaints: usend suppresses addresses that hard-bounce or complain. See Bounces and suppressions. Customer.io will not see usend's suppression list; subscribe to webhooks if you need to sync it back.
• Recipients per message: up to 50.
• Tracking: if Customer.io tracks opens and clicks itself, leave usend's tracking off to avoid double-wrapping links. See Open and click tracking.
• Warm-up: do not move a large list at once. See Warm up a new domain.

If you only need usend for some messages, keep the others on your current provider and move them in stages.