python-send-mail

btx_lib_mail - send email from Python or the shell, streamed

The btx_lib_mail repo is itself a Claude Code plugin/marketplace. Install this skill in any project with /plugin marketplace add bitranox/btx_lib_mail then /plugin install btx_lib_mail. It is also mirrored in the central bitranox marketplace (https://github.com/bitranox/bitranox-skills) as coding-python-send-mail.

When to reach for this (and what to avoid)

Need	Use	Avoid
Send mail from Python (multipart, UTF-8, attachments)	btx_lib_mail.send(...)	hand-rolling smtplib + email MIME assembly
Send a large attachment on a low-memory box	send(..., attachment_file_paths=[...]) (streamed)	reading the file into RAM, sendmail(msg_str)
Send from a shell / an agent with nothing installed	uvx btx-lib-mail send ...	installing a mailer, writing a throwaway script
STARTTLS + auth, multi-host failover	send(..., use_starttls=True, credentials=...)	rewriting the connect/login/failover loop
Refuse dangerous or sensitive attachments	built-in attachment security (on by default)	ad-hoc path checks

Install / run

Zero-install (best for agents and one-offs): run the CLI straight from PyPI. Nothing is installed persistently.

uvx btx-lib-mail --help
uvx btx-lib-mail send --host smtp.example.com:587 \
  --sender [email protected] --recipient [email protected] --subject "Hi" --body "hello"

Add to a project, or install the CLI on PATH:

uv add btx_lib_mail            # or: pip install btx_lib_mail
uv tool install btx_lib_mail   # or: pipx install btx_lib_mail  (CLI on PATH)

Requires Python 3.10+. Both btx_lib_mail and btx-lib-mail are registered commands; python -m btx_lib_mail runs the same CLI.

Library usage

from btx_lib_mail import send

send(
    mail_from="[email protected]",
    mail_recipients=["[email protected]"],  # str or sequence; validated, deduped
    mail_subject="build failed",  # UTF-8 is fine (Grüße, emoji, CJK)
    mail_body="See CI logs.",
    mail_body_html="<p>See CI logs.</p>",  # optional HTML alternative
    smtphosts=["smtp.example.com:587", "smtp-dr.example.com:587"],  # tried in order (failover)
    credentials=("user", "pass"),  # optional
    use_starttls=True,  # default True, verifies the cert by default
)

send returns True when every recipient is accepted, and raises RuntimeError when all hosts fail for a recipient. Set global defaults on conf and override per call:

from btx_lib_mail import conf

conf.smtphosts = ["smtp.example.com:587"]
conf.smtp_username = "mailer"
conf.smtp_password = "s3cr3t"  # SecretStr; a plain str is coerced. Per-call kwargs override conf.

Large attachments (streamed, bounded memory)

Attachments are streamed from disk and sent to the server in chunks, so a multi-gigabyte file never has to fit in RAM. The trade is temporary disk, not memory: a huge attachment needs scratch disk of about 1.33x its size (base64), never that much RAM.

from pathlib import Path
from btx_lib_mail import send

send(
    mail_from="[email protected]",
    mail_recipients="[email protected]",
    mail_subject="nightly dump",
    mail_body="Attached.",
    smtphosts=["smtp.example.com:587"],
    attachment_file_paths=[Path("/data/backup-20GB.tar")],
    attachment_max_size_bytes=None,  # REQUIRED for big files: the default cap is 25 MiB
)

The default attachment_max_size_bytes is 25 MiB, so a large file is rejected until you raise the cap or set it to None. The server's own SIZE limit still applies.

CLI

Pass the password via the BTX_MAIL_SMTP_PASSWORD environment variable, never as --password on the command line (a literal argv value leaks into shell history and ps output):

BTX_MAIL_SMTP_PASSWORD="$(cat /path/to/credential_file)" \
uvx btx-lib-mail send \
  --host smtp.example.com:587 \
  --sender [email protected] \
  --recipient [email protected] \
  --subject "Ping" --body "Smoke test" \
  --attachment /data/report.pdf \
  --username user \
  --starttls \
  --attachment-max-size 5000000000    # raise the 25 MiB default for a large attachment

Other commands: info, hello, validate-email, validate-smtp-host. Every option also reads a matching BTX_MAIL_* environment variable (for example BTX_MAIL_SMTP_HOSTS, BTX_MAIL_RECIPIENTS, BTX_MAIL_SMTP_PASSWORD). Run uvx btx-lib-mail send --help for the full option list and precedence.

Streaming and BDAT (how delivery works)

• The message is composed once into a disk-backed spool and streamed to the socket in fixed-size chunks, so peak memory is roughly one chunk regardless of attachment size.
• When the server advertises CHUNKING (RFC 3030) the body is sent as length-prefixed BDAT chunks; otherwise the classic DATA phase is used with dot-stuffing. This is automatic per host.
• STARTTLS and authentication happen before either path. Certificate verification is on by default; opt out for an internal self-signed relay with starttls_verify=False (or --no-starttls-verify), which keeps the channel encrypted but skips validation.

Attachment security

Attachments are validated before any bytes are read, and rejected for: path traversal (..), symlinks (off by default), sensitive patterns (/.ssh/, /id_rsa, /.env, credentials), system directories, dangerous extensions (.sh, .exe, and the like), and oversize payloads. Violations raise AttachmentSecurityError by default, or log-and-skip with attachment_raise_on_security_violation=False. There are whitelist modes (attachment_allowed_extensions, attachment_allowed_directories). Because dangerous extensions and system directories are blocked by default, pass attachment_blocked_extensions=frozenset() (or an allowlist) when you deliberately send such a file.

Reference

The API and CLI surface is discoverable from the INSTALL (always matches your version): run uvx btx_lib_mail --help for every CLI option, and python -c "import btx_lib_mail as m; help(m)" for the public API - send, conf, ConfMail, validate_email_address, validate_smtp_host, and the attachment-security constants, all re-exported from the package root.

Narrative detail (every ConfMail field, env-var precedence, streaming, attachment security) lives in the repo docs (NOT shipped in the pip wheel), on the default branch so they track the latest release you get from uv: https://github.com/bitranox/btx_lib_mail/blob/master/README.md and, under https://github.com/bitranox/btx_lib_mail/blob/master/docs/, the files api.md, cli.md, configuration.md, streaming.md, attachment-security.md.