Limits & Behaviour - MailBreeze

Limits

Limit	Value
Maximum message size	25 MB, including attachments
Recipients per message	50 across To, Cc, and Bcc
Messages per connection	100
Concurrent connections per IP	10
Failed authentications before lockout	10 in 15 minutes

Exceeding the recipient cap rejects the additional recipients and delivers to the ones already accepted, rather than failing the whole message.

Recipients

The envelope determines delivery; headers determine what recipients see.

Addresses in the To header are delivered to and shown as To.
Addresses in the Cc header are delivered to and shown as Cc.
Envelope recipients that appear in neither header are treated as Bcc and are never written into any header.

A recipient rejected during the SMTP conversation is not delivered to, even if it still appears in a header.

Every message needs at least one To or Cc recipient. Bcc-only messages are rejected:

550 At least one To or Cc recipient is required; Bcc-only messages are not accepted

To send to a group privately, address the message to yourself in To and put the group in Bcc.

Headers

MailBreeze generates the headers that identify and authenticate a message: Message-ID, Date, DKIM-Signature, Return-Path, Feedback-ID, and the unsubscribe headers. Any copy you supply is replaced. Preserved from your message:

Header	Purpose
`In-Reply-To`	Keeps replies threaded in the recipient’s client
`References`	Keeps replies threaded in the recipient’s client
`X-*`	Your own custom headers

Headers outside that list are removed. This includes Errors-To, Return-Receipt-To, Sender, Precedence, Auto-Submitted, and List-Unsubscribe, which would otherwise interfere with bounce processing and deliverability. The X-MB-* prefix is reserved for MailBreeze and is stripped from incoming messages.

Tracking

Open and click tracking are enabled by default, matching the API. Disable either per message with a header:

X-MB-Track-Opens: false
X-MB-Track-Clicks: false

These two headers are read and then removed, so they never reach the recipient.

Idempotency

If your client supplies a Message-ID, MailBreeze uses it to deduplicate for 24 hours. A message retried after a temporary failure is delivered and billed once, and both attempts return the same message ID. Clients that generate a fresh Message-ID per attempt cannot be deduplicated, so retries will send twice.

What SMTP cannot do

These features are API-only:

Templates

Server-side rendering with templateId and variables.

Stored attachments

Referencing previously uploaded attachments by ID. Attach files to the message instead.

Marketing sends

Campaigns and list sends. SMTP messages are always transactional.

Scheduling

Delayed or scheduled delivery.

​Limits

​Recipients

​Headers

​Tracking

​Idempotency

​What SMTP cannot do