Mailpit is an email testing tool and API for developers. From 1.30.0 until 1.30.5, Mailpit's internal/smtpd/smtpd.go readData() function calls bufio.Reader.ReadBytes before applying the len(data)+len(line) size check to the completed SMTP DATA line against Server.MaxSize. An unauthenticated SMTP client can send a single line larger than the configured MaxMessageSize, causing the full line to be allocated before Mailpit returns the 552 5.3.4 rejection. This post-fix gap remains after normal multi-line DATA accumulation was bounded, and concurrent oversized lines can create substantial memory pressure beyond the configured message-size cap. This issue is fixed in version 1.30.5.

Affected: Go: github.com/axllent/mailpit

## Summary

Mailpit's SMTP DATA reader enforces the configured `MaxMessageSize` only after `bufio.Reader.ReadBytes('\n')` has already buffered a complete DATA line. A remote unauthenticated SMTP client can send one line larger than the configured message-size cap and force memory allocation before Mailpit returns the expected `552 5.3.4` rejection, leaving patched versions still exposed to a single-line incomplete-fix variant of the earlier SMTP DATA body-size issue.

## Technical Details

Mailpit enables SMTP by default. The SMTP server now wires `config.MaxMessageSize` into `srv.MaxSize`:

```go if config.MaxMessageSize > 0 { srv.MaxSize = config.MaxMessageSize * 1024 * 1024 } ```

The DATA reader then checks that cap, but only after reading a full newline-terminated line into memory:

```go line, err := s.br.ReadBytes('\n') if err != nil { return nil, err }

if bytes.Equal(line, []byte(".\r\n")) { break } if line[0] == '.' { line = line[1:] }

if s.srv.MaxSize > 0 { if len(data)+len(line) > s.srv.MaxSize { _, _ = s.br.Discard(s.br.Buffered()) return nil, maxSizeExceeded(s.srv.MaxSize) } } ```

This ordering violates the size-limit invariant. The configured cap can reject the message only after the attacker has supplied the line terminator and `ReadBytes('\n')` has allocated the over-limit line. With the default 50 MiB cap, a 64 MiB single DATA line is still buffered before Mailpit returns `552 5.3.4 Requested mail action aborted: ex…