Proton Mail SMTP Settings

SMTP is the protocol your mail client uses to send messages. It is configured entirely separately from the incoming connection, which is why an account can receive mail perfectly while every outgoing message sits stubbornly in the outbox. The two halves are independent, and when sending fails it is almost always only the outgoing half that is wrong.

Proton Mail SMTP settings

These go in the outgoing server section of your mail client:

Authentication is required. Older clients sometimes default to leaving the outgoing server unauthenticated, on the assumption that anyone on the network is trusted — an assumption no modern mail provider accepts. If your client has a separate "outgoing server requires authentication" checkbox, it needs to be ticked, and it should generally use the same credentials as the incoming server.

Before you start

• Requires: Proton Mail Bridge, available on paid plans only
• Why localhost: Bridge runs on your machine and decrypts locally, so clients connect to 127.0.0.1 rather than a Proton server
• Credentials: Bridge issues its own username and password — not your Proton login

Choosing the right port

Port 587 with STARTTLS is the modern standard. The connection opens unencrypted and then immediately upgrades to TLS before any credentials are exchanged, so nothing sensitive travels in the clear. Port 465 with SSL/TLS takes the other approach and encrypts from the very first byte. Both are secure; the choice usually comes down to which one your client handles more gracefully.

Port 25 is a different matter. It exists for server-to-server delivery, not for mail clients, and virtually every home broadband provider, mobile network and hotel connection blocks it outright to limit spam. If a guide anywhere tells you to configure port 25 in a desktop mail client, that guide is out of date.

When mail will not send

Start with authentication, since it is the most common culprit: confirm the outgoing server is set to authenticate and is using credentials that actually work for incoming mail. Then check the port and encryption pairing against the table above, because a mismatch here produces errors that read like network faults.

If both look right, try sending from a mobile hotspot instead of your usual connection. A message that sends instantly on mobile data but times out on your office or hotel network tells you the network is blocking outbound mail ports, not that your settings are wrong — and no amount of reconfiguring the client will fix that. On a corporate network, the answer is usually to ask whoever runs it to allow the port.

Sending limits

Most providers cap how many messages an account may send in a day, and how many recipients a single message may address. These limits exist to contain compromised accounts, and they apply to mail sent through SMTP exactly as they do to mail sent from the website. Hitting one typically produces a temporary rejection rather than a permanent failure, and normal service resumes once the window rolls over. If you regularly need to send in bulk, a dedicated transactional mail service is the right tool rather than a personal mailbox.