ESP
Email service provider — the tool that sends your campaigns (e.g. Klaviyo, Mailchimp, Braze).
Say it out loud
“The tool we send from. Klaviyo, in our case. It is not the same thing as our website or our inbox.”
An ESP (email service provider) is the platform where contacts live, campaigns are built, and sends are executed. It is not your website host, not your DNS provider, and not the mailbox your recipients read in. Confusing those four is the most expensive vocabulary error in this job.
Klaviyo, Mailchimp, Braze, HubSpot, Salesforce Marketing Cloud.
Cloudflare, GoDaddy, Route 53. Where the proof of identity is published.
Gmail, Outlook, Apple Mail. They decide, and they do not tell you why.
No platform has ever fixed this one.
An afternoon spent in the wrong one of these four is the most common wasted afternoon in email marketing.
Nobody's
Nothing to action — it changes a number or a risk you report.
What goes wrong
Escalating a spam-folder problem to ESP support. They can see that they handed the message over successfully, which is nearly always true and nearly never the answer.
See also
Where this sits
Stop 2, you build the message. You choose a From address, write a subject line, and add the things the law requires to be in the message.