Email API vs SMTP for UK Teams

Both routes do the same basic job: they let software send email through Mailpro rather than through your own mail server. The difference is how your software talks to Mailpro and how much it gets back. For a UK business the decision usually comes down to one question: what can the system that needs to send actually support?

The short version

When the API is the better choice

Choose the email API when you control the code. A London fintech sending account alerts, a subscription box company in Bristol sending dispatch notices or a booking platform sending confirmations will all benefit from:

• structured requests with templates and personalisation fields;
• an identifier for every send, so you can check whether it was delivered, deferred or bounced;
• contact and list management from the same integration, keeping marketing consent in sync with your database;
• webhooks that notify your system about bounces and other events.

When SMTP is the better choice

Choose SMTP when the sending software is not yours to change. Many UK businesses run accounting, practice-management or ERP software that emails invoices and statements. Those tools typically have an SMTP settings screen and nothing else. Pointing them at Mailpro’s SMTP server takes minutes and improves deliverability compared with sending from a shared office mailbox. The FAQ on which SMTP port to use helps with the settings.

What stays the same either way

• Authentication: set up SPF, DKIM and DMARC on your domain before going live.
• Encryption: Mailpro uses TLS 1.3 for connections.
• Data location: messages and logs are handled on Mailpro’s private Swiss infrastructure, outside US jurisdiction. The UK recognises Switzerland as adequate for data transfers.
• The law: PECR treats a message as marketing based on its content, not how it was sent. An invoice sent by SMTP is a service message; a promotional email sent by API still needs consent or the soft opt-in.

Three UK scenarios

An accountancy practice

The practice’s software emails engagement letters, invoices and reminders about the 31 January Self Assessment deadline. It has an SMTP settings page and no API. Answer: SMTP. The practice gains authenticated sending from its own domain and a record of what was sent, which helps when a client says the reminder never arrived.

A direct-to-consumer brand

The shop sends order confirmations, dispatch notices and delivery updates, and wants marketing consent captured at checkout to flow into its newsletter list. Answer: the API, because one integration handles both transactional sends and contact sync.

A membership charity

The charity’s CRM sends renewal reminders by SMTP, while its developers are building a new supporter portal. Answer: SMTP now for the CRM, the API for the portal, both on the same Mailpro account.

Security checklist for either route

• Store API keys and SMTP passwords in a secrets manager, never in source code or shared documents.
• Use separate credentials per system, so you can revoke one without breaking the others.
• Restrict who in your team can see sending logs, since they contain personal data covered by UK GDPR.
• Monitor bounces and complaints, and alert someone when they rise.

Using both

Plenty of organisations use both. A UK online retailer might send order confirmations through the API from its shop, invoices through SMTP from its accounting software, and newsletters through the Mailpro interface, all from one account and one sending domain. Mailpro’s article on SMTP relay vs email API goes deeper into the technical differences, and the SMTP and API FAQ answers common questions.

Still unsure?

Start with whatever your system supports today. If you later build a custom integration, moving transactional sends from SMTP to the API doesn’t require a new account or a new domain set-up. Both are priced by sends, not contacts.