Sending e-mail by SMTP or through an e-mail API: which suits your application

For a few messages (a contact form, the confirmations of a small shop), SMTP with a mailbox on your domain is enough. For large volumes, newsletters, or when you need to know what happened to each message, use a sending service through its API. Many of those services offer both routes, so the choice is rarely “one or the other” for ever.

What each route is

SMTP is the protocol of e-mail itself: the application connects to a server with a user name and password and hands over the message. An API is an HTTPS request to a third-party sending service (one you contract separately): the application sends the message data and the service does the rest. Behind the scenes the service also sends by SMTP; what changes is how you hand it the message.

Side by side

Criterion SMTP The service’s API
How it connects Server, port, user name and password. Almost all software knows how. An HTTPS request with a key. Needs code, or a plugin that talks to the service.
Effort to start Low: you fill in a form in the settings. Medium: you write code, or install a plugin.
What you get to know Only whether the server accepted the message. Usually also what happened afterwards (delivered, bounced, complaint), through notices the service sends.
Volume Limited by the mailbox and plan limits. Built for large volumes, under the service’s rules.
Reputation Shares that of your domain and the server. Depends on the service and on what you send through it.
Where it fits WordPress, forms, ready-made programs. See making WordPress send e-mail with SMTP. Applications of your own, with heavy sending, or where each message counts.

How to decide

1 How many messages a day? Dozens, SMTP. Thousands or campaigns, a service of its own. The exact sending limits on your account are in how to write an e-mail that does not land in spam: behaviour, recipients and limits.
2 Do you need proof of delivery? Invoices and receipts by e-mail that the customer must receive justify a route that tells you what happened to each message.
3 Who writes the code? If nobody programs, use SMTP: it is a setting, not code. If you do program, see sending e-mail from PHP: mail() or SMTP and PHPMailer.
4 Do you have a newsletter? Do not send it from your everyday mailbox. See e-mail marketing without burning your domain.

What does not change on either route

The domain the message comes from must be authenticated. With a third-party service, it gives you SPF and DKIM records to put in DNS. A domain should have only one SPF record: merge the authorisations into the same record, do not create a second. See SPF, DKIM and DMARC.

An API key is a password. Whoever has it sends e-mail in your name. Keep it in an environment file, never in public code or in a repository, and replace it if you suspect it leaked. See keeping passwords out of your PHP code.
Third-party services are contracted separately. We do not sell them; the setup on your side (DNS, plugins, code) is yours, and support explains how far it goes in how far our support goes.

Not sure whether what you want to send fits in a mailbox? Tell us the kind of messages and the volume.

Open a support ticket

SEE ALSO

Sending e-mail from PHP: mail() or SMTP

Making WordPress send e-mail properly, with SMTP

SPF, DKIM and DMARC: why your e-mail lands in spam

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