Send Mastodon email through SMTP with Ackuna

Before you start

You need three things from Ackuna. Each takes a few minutes, except the review in step 2.

1. An Ackuna account. Create one for free; no card is needed.
2. A verified sending domain. Add the domain your app sends from (for example yourdomain.com) under SMTP Relay, Domains in the console and publish the SPF, DKIM and DMARC records it shows you. The console checks them for you. A new sending domain is also reviewed by our team before it can send; that is not instant, and until it is approved the relay answers 452.
3. An SMTP credential for that domain. Under SMTP Relay, Credentials, create one for the verified domain. You get a username (smtp_…) and a password (sc_…). The password is shown once, so paste it into Mastodon straight away; if you lose it, rotate the credential.

The From address in Mastodon must be on exactly that domain. A credential for yourdomain.com can send as [email protected], but not from a subdomain such as mail.yourdomain.com or from another domain.

The settings

Mastodon reads its configuration from .env.production in the Mastodon directory. Set the e-mail variables like this:

SMTP_SERVER=smtp.ackuna.com
SMTP_PORT=587
SMTP_LOGIN=YOUR_SMTP_USERNAME
SMTP_PASSWORD=YOUR_SMTP_PASSWORD
[email protected]
SMTP_AUTH_METHOD=plain
SMTP_ENABLE_STARTTLS=auto
SMTP_TLS=false
SMTP_SSL=false

• Mastodon tries STARTTLS by default, and SMTP_ENABLE_STARTTLS=auto is that default written out. The relay always offers STARTTLS on port 587, so the connection is encrypted before the login.
• SMTP_TLS and SMTP_SSL are TLS on connect. Both default to false; keep them that way for port 587.
• SMTP_AUTH_METHOD=plain is the default. Mastodon's docs note that plain transmits the password in the clear; the relay accepts a login only after the STARTTLS upgrade, so the password travels encrypted.
• SMTP_FROM_ADDRESS must be on your verified domain.

Apply the change

Restart Mastodon so it reads the new values. On an install from source, with the systemd services from Mastodon's installation guide:

sudo systemctl restart mastodon-web mastodon-sidekiq

Send a test email

Mastodon has no test-mail button in its documentation. Trigger a real email: request a password reset from the login page for an account whose address you can read, or sign up a test account. Then check SMTP Relay, Messages in the Ackuna console, which lists the message and whether it was delivered.

Common errors

Authentication failed (535)

The username or password is wrong, the credential was revoked, or its domain is no longer active. Copy both values again from the console, with no spaces around them. If you no longer have the password, rotate the credential and paste the new one.

TLS handshake error, "wrong version number" or a connection that hangs

The app is trying SSL/TLS on connect, which the relay does not offer. Use port 587 with STARTTLS: the connection starts in plain text and is upgraded before the login. If the connection times out instead, check that your host or firewall allows outgoing connections to port 587; some cloud providers block it on new servers until you ask them to lift the block.

Sender rejected (550)

The From address is not on the domain the credential was created for. Set the From address in Mastodon to an address on that exact domain. The same code is used for a recipient on your suppression list, because an earlier message to them bounced or was marked as spam.

Temporarily rejected (452)

Either you have reached your daily or monthly limit, or the sending domain is still waiting for review. Both are temporary: a mail server retries later, and the console shows your usage. A message with more than 10 recipients is also refused with 452, and one over 10 MB with 552.

Mail that is accepted but lands in spam usually points at DNS, not SMTP. Run the Email DNS check on your domain, or the SPF, DKIM and DMARC checkers one by one.

Limits

• The Free plan includes 3,000 relay emails a month and 100 a day, 1 SMTP credential and 1 sending domain. Paid plans and their limits are on the pricing page.
• New accounts start on lower limits until Ackuna has reviewed them: at most 20 Agent Mail, 200 SMTP relay and 500 newsletter emails a day, whatever the plan allows. After the review the plan limits above apply.
• Each new sending domain is reviewed before it can send.
• One message can have up to 10 recipients and be up to 10 MB.
• While we deploy an update, port 587 stays open and the relay holds the mail it accepts, then sends it when the update is done. A login it cannot check during that time gets a temporary 451; a retry goes through once the update is done.