mailcat

May 23, 2026 · View on GitHub

The only cat who can find existing email addresses by nickname.

Install

From PyPI (recommended):

pip install mailcat-osint

From source:

git clone https://github.com/sharsil/mailcat
cd mailcat
pip install .

Usage

Run it with a username:

mailcat username

Or as a module:

python -m mailcat username

It's recommended to run mailcat through Tor or a proxy — see Routing through Tor below.

mailcat --tor username
proxychains4 -q mailcat username

mailcat username --proxy http://1.2.3.4:8080

Batch mode

You can check multiple usernames in one run.

Pass several usernames as positional arguments:

mailcat alice bob charlie

Or supply a file with one username (or email address) per line:

mailcat --file usernames.txt

File format example:

alice
bob@proton.me
charlie

You can also combine both — positional names are merged with those from the file:

mailcat alice --file more_users.txt

When more than one username is resolved, a header is printed before each result block so the output is easy to follow:

[*] Checking username: alice
...
[*] Checking username: bob
...

26 active providers covering > 158 domains (plus ~50 Posteo alias domains). Active checks run by default; deprecated checks remain in the source for revival but are skipped on a default run. See the comment block above each function in src/mailcat/__init__.py for the upstream change that broke it and notes on how to revive it.

Name	Domains	Method	Status
Gmail	gmail.com	SMTP	Active
Yandex	yandex.ru + 5 aliases	SMTP	Active
Protonmail	protonmail.com + 3 aliases	API	Active
MailRu	mail.ru + 4 other domains	Registration	Active
Rambler	rambler.ru + 5 other domains	Registration	Active
Yahoo	yahoo.com	Registration	Active
AOL	aol.com	Registration	Active
Outlook	outlook.com, hotmail.com, live.com, outlook.de, msn.com	MSA credential lookup	Active
Zoho	zohomail.com	Registration	Active
Eclipso	eclipso.eu + 9 other domains	Registration	Active
Posteo	posteo.net + ~50 aliases	Registration	Active
Firemail	firemail.de + 2 other domains	Registration	Active
Fastmail	fastmail.com	Headless Chromium	Active
StartMail	startmail.com	Registration	Active
Ukrnet	ukr.net	Registration	Active
Runbox	runbox.com + 29 other domains	Registration	Active
DuckGo	duck.com	Registration	Active
emailn	emailn.de	Registration	Active
aikq	aikq.de + 40 other domains	Registration	Active
Vivaldi	vivaldi.net	Registration	Active
mailDe	mail.de	SMTP	Active
int.pl	int.pl	Headless Chromium	Active
Interia	interia.pl + 10 other domains	Password recovery	Active
t.pl	t.pl + 8 other domains	Registration	Active
onet.pl	onet.pl + 15 other domains	Headless Chromium	Active
Mailum	cyberfear.com, mailum.com	Registration	Active
iCloud	icloud.com, me.com, mac.com	Account recovery	Deprecated
HushMail	hushmail.com + 5 other domains	Registration	Deprecated
Xmail	xmail.net	Registration	Deprecated
Tutanota	tutanota.com + 4 other domains	Registration	Deprecated
Mailbox.org	mailbox.org	Registration	Deprecated
WP	wp.pl	Registration	Deprecated
O2	o2.pl	Registration	Deprecated
Gazeta.pl	gazeta.pl	Registration	Deprecated

Troubleshooting

Use -m or --max-connections if you get connection errors (Mailcat does 10 parallel connections max by default).

Routing through Tor

The SMTP-based checks (gmail, yandex, mailDe) reach the destination provider's MX servers on TCP port 25. Most residential ISPs and every major cloud provider (AWS, GCP, Azure, Heroku, …) block outbound port 25 as an anti-spam measure, so these checks will time out with messages like:

Error while checking gmail: Timed out connecting to gmail-smtp-in.l.google.com. on port 25

Tor exit nodes generally do not have port 25 blocked, which makes routing through Tor the easiest fix. Mailcat has built-in Tor support via the --tor flag — it expects a SOCKS5 proxy at 127.0.0.1:9050 (the default when you run tor locally).

# 1. Start Tor (macOS / Linux)
brew install tor && tor &        # macOS
sudo systemctl start tor          # Linux with systemd

# 2. Run mailcat through Tor — SMTP checks now succeed
mailcat alex --tor

# Or restrict to just the providers that need it
mailcat alex --tor -p gmail -p yandex -p mailDe

Tor adds ~5–15 s of latency per request, but it is the only reliable way to make the SMTP-25 checks work from a typical home or cloud environment. A clean VPS with unblocked egress 25 (Hetzner after verification, OVH, Vultr) works equally well — point mailcat at it via --proxy or just run mailcat on the VPS itself.

The HTTP-based checks (everything else) work fine from any network and do not require Tor.

SOWEL classification

This tool uses the following OSINT techniques:

Mentions and articles

OSINTEditor Sunday Briefing: Sensational Headlines and Kuomintang Chairmanship Elections

Michael Buzzel: 237 - The Huge OSINT Show by The Privacy, Security, & OSINT Show

bellingcat: First Steps to Getting Started in Open Source Research

OS2INT verifying email addresses using Mailcat

hwosint - Twitter post