SMTP, the Simple Mail Transfer Protocol, is the standard that email servers use to pass messages to one another. Every email you send, whether a newsletter to subscribers in Quebec or a receipt from a store in British Columbia, travels across the internet using SMTP.
The journey of one email
- Your application or email client hands the message to an SMTP server, after logging in with a username and password.
- The server looks up the recipient domain’s MX record in DNS to find where to deliver it.
- It connects to the recipient’s mail server and transfers the message, ideally over an encrypted TLS connection.
- The recipient server runs checks, including SPF, DKIM and DMARC, spam filtering and reputation, then places the message in the inbox, the spam folder, or rejects it.
- The recipient reads it with a separate protocol such as IMAP or POP3.
Ports you will see
- 587: the standard submission port, with STARTTLS encryption. Use this for most applications.
- 465: submission over implicit TLS, supported by many services.
- 25: server-to-server relay. Many residential and business internet providers block outbound port 25 to reduce spam, so applications rarely use it directly.
Authentication records
Mailbox providers now expect bulk senders to prove who they are. Three DNS records do that: SPF lists which servers may send for your domain, DKIM signs each message cryptographically, and DMARC tells receivers what to do if the first two fail. Setting them up is the single most effective step for deliverability.
Running your own server vs using a relay
You can run your own SMTP server, but you then manage IP reputation, blocklists, bounces, encryption and updates. For most Canadian small and medium businesses, a managed relay is simpler and more reliable. Mailpro’s relay also offers deliverability routing, sending mail through the most appropriate path for each destination.
Bounces and response codes
Receiving servers answer every SMTP transaction with a code. Codes starting with 2 mean success; 4 means a temporary problem, such as a full mailbox, and the sending server will retry; 5 means a permanent failure, such as an address that doesn’t exist. A good relay reads these codes for you, retries temporary failures and flags permanent ones so you can remove them. See our pages on hard bounces and soft bounces.
SMTP and Canadian law
SMTP is just the transport. CASL governs the content: commercial messages need consent, sender identification and an unsubscribe mechanism, while purely transactional messages are largely exempt from the consent requirement. On the privacy side, the SMTP servers that handle your mail also process personal information, so under PIPEDA you remain accountable for the provider you choose, and Quebec’s Law 25 asks for an assessment before data leaves the province. Mailpro’s SMTP infrastructure runs in Switzerland, outside US jurisdiction, and uses TLS 1.3.
Where to go next
To start sending through a managed relay, see the SMTP server for Canada page. If you’re writing new software, compare it with an API on API vs SMTP. For the full product details, visit Mailpro SMTP, and check volumes on the pricing page.