Running outbound mail from a shared hosting IP is a reputation game that most providers lose slowly. One compromised contact form can undo months of clean sending, and getting an IP off a major blocklist costs engineering hours every time. MailBaby is InterServer’s answer to that problem: an outbound-only SMTP smarthost that takes every message your server sends, scores it, and delivers it from IP pools whose reputation InterServer manages. This guide explains what the service does, what it costs, and the constraints you need to design around before pointing a production server at it.
Table of Contents
Short answer: MailBaby is InterServer’s outbound-only SMTP smarthost: a hosting server authenticates to relay.mailbaby.net on port 25 with STARTTLS, every message is spam-scored, and clean mail is delivered from reputation-managed IP pools while bad mail is bounced back. It costs $1 per month for about 5,000 messages plus $0.20 per 1,000 after that, caps sending at 6,000 messages per hour per address, and leaves inbound mail, MX records and Dovecot untouched.
What the service actually does
MailBaby sits between your mail server and the internet. Exim, Postfix or Plesk authenticates to relay.mailbaby.net on port 25 with STARTTLS, hands over the message, and MailBaby takes responsibility for delivery. Every message passes through a content filter that assigns a spam score. Clean mail is routed out through a sending pool matched to the reputation of the sending account; borderline mail goes through a lower-reputation pool; mail that scores badly is bounced back to your server with a rejection such as “classified as rSPAM” so the sender knows it was not delivered.
Because InterServer has run the platform on its own infrastructure since May 2024, the scoring model, the IP pools and the feedback-loop handling are all in-house. That matters for support: when a message is blocked, the log entry and the score are visible in the customer portal rather than hidden inside a third-party filter.
The service handles the tedious parts of outbound deliverability on your behalf: rDNS on the sending IPs, feedback-loop registration with the large mailbox providers, bounce parsing, and SRS rewriting for forwarded mail. Inbound mail is not part of the product. Your MX records, Dovecot and inbound spam filtering stay exactly where they are.
Pricing and what gets billed
The account costs $1 per month, which includes roughly 5,000 messages. Beyond that, sending is billed at $0.20 per 1,000 messages. There is no per-server or per-domain fee, so a provider with forty cPanel servers pays for volume alone. Two billing details catch people out:
- Messages that MailBaby blocks or bounces as rSPAM still count as sent. A compromised account spewing 20,000 rejected messages is a real cost, not a free rejection.
- The $1 base covers a single account. Most providers use one account per server or per reseller so that logs and limits are separable; each is its own $1.
Ordering is done through the InterServer customer portal at my.interserver.net, where you also find the SMTP username (mbXXXXX style, with no domain part), the password, the logs and the block management tools.
Limits you must design around
The hard ceiling is 6,000 messages per hour per sender address. Excess messages are discarded rather than queued, so a newsletter script that pushes 10,000 messages from one address in a burst loses 4,000 of them silently at the relay. Throttle at the source: cPanel’s per-domain hourly limits, Postfix’s default_destination_rate_delay, or the sending application’s own batch settings. We set the server-side cap below 6,000 so that the relay limit is never the one that fires.
New accounts are treated more strictly during a warm-up period. Expect a lower effective threshold and more aggressive scoring for the first days of sending. Migrating a busy server onto a brand-new account on a Friday evening is a bad idea; move a low-volume server first.
Logs are retained for 60 days and record the sender, recipient, subject and spam score. Message bodies are removed once delivery completes, so MailBaby cannot recover the content of a message for you later.
STARTTLS is required on the inbound connection from your server. Plain-text authentication on port 25 without TLS is refused, which is why every configuration example on this site sets hosts_require_tls or smtp_tls_security_level to at least opportunistic TLS.
Prohibited content
The acceptable use policy excludes several categories regardless of how well the mail is formatted: cold or unsolicited outreach, gambling, cryptocurrency promotion, pharmaceutical sales and debt collection. These are not scored on merit; a customer in one of these verticals will be blocked and can get your account suspended. Screen new signups against this list before you route their mail through the relay, and route legitimate high-risk senders through a dedicated IP elsewhere.
Where it fits in a hosting stack
A typical deployment keeps the panel’s mail stack unchanged and adds a smarthost route. On cPanel that is an Exim authenticator, a manualroute router and an SMTP transport with hosts_require_auth, described in our cPanel/WHM setup guide. DirectAdmin uses the exim.*.pre.conf and exim.*.post.conf override files, covered in the DirectAdmin guide. Postfix servers need a relayhost plus SASL credentials, covered in the Postfix guide.
DNS needs one change per customer domain: either add include:spf-c.mailbaby.net to the SPF record or publish a _mailbaby.<domain> verification TXT record. The two methods are compared in the SPF and verification guide. DKIM signatures generated on your server pass through untouched, which is the arrangement you want; without them MailBaby signs with its own domain and recipients see “via mailbaby.net”.
Verify before going live
Send a test from a mailbox on the server to an external address and read the headers of the received message. The Received: chain should show your server handing off to relay.mailbaby.net, and Authentication-Results should report spf=pass, dkim=pass for your domain and, if you publish a policy, dmarc=pass. In the InterServer portal, confirm the message appears in the log with a low score. Then watch the log for the first 24 hours: a sudden run of rSPAM rejections from one address is the earliest sign of a compromised customer account, and the relay will show it before your local Exim log does.
What is MailBaby at a glance
Official documentation: RFC 5321 (SMTP), Linux man pages.
Related guides: Newsletters and mailing lists through MailBaby: staying under the 6,000/hour limit and out of spam folders · MailBaby DKIM transport signing explained: why some mail shows “via mailbaby.net” · Sending WordPress email through MailBaby: SMTP plugin setup and testing.
Frequently asked questions
Does MailBaby handle inbound email as well as outbound?
No. MailBaby is outbound only. Your MX records, Dovecot, inbound spam filtering and mailbox storage stay on your own server; the relay only takes messages your server sends and delivers them from its IP pools.
Is MailBaby worth it for a small hosting server?
Usually yes. At $1 per month plus $0.20 per 1,000 messages beyond the first 5,000, a server sending a few thousand messages a month costs almost nothing, and the saving is the engineering time otherwise spent on blocklist removals and IP warm-up.
What happens if a customer exceeds the 6,000 messages per hour MailBaby limit?
Messages over the cap are discarded at the relay rather than queued, so they are lost silently. Set a lower per-domain hourly limit on the server so that Exim or Postfix queues the excess locally and the relay limit is never the one that triggers.