Swaks
Swaks is the "Swiss Army Knife for SMTP". First released in 2003, it has become a go-to tool for testing mail servers and diagnosing SMTP connection issues, and there's very little it can't do. It's particularly useful for verifying server details and credentials, and sending quick tests.
All the available options can be a little overwhelming though, so here are some common useful examples.
Installation
Swaks is available in most package managers, so installing Swaks on macOS or Linux is a breeze:
macOS
- Homebrew -
brew install swaks - MacPorts -
port install swaks
Linux
- Debian / Ubuntu -
sudo apt-get install swaks - Fedora -
sudo dnf install swaks - Arch -
sudo pacman -S swaks
Windows
There's no packaged version for Windows, however installing Windows Subsystem for Linux (WSL) and then following the Linux installation instructions may be the easiest way.
Send a simple test email
This example will send a simple plain text email with a subject such as test Fri, 04 Jul 2025 11:54:51 +0000, and a message body of This is a test mailing
swaks \
--from [email protected] \
--to [email protected] \
--server in.smtp.sendamatic.net:587 \
--auth plain \
--tls \
--auth-user [mail credential user] \
--auth-password [mail credential password]
Send attachments
Attaching files can be done with the --attach option, which can be added multiple times.
swaks \
--from [email protected] \
--to [email protected] \
--server in.smtp.sendamatic.net:587 \
--auth plain \
--tls \
--auth-user [mail credential user] \
--auth-password [mail credential password] \
--h-Subject 'Attachment test' \
--body 'Attachment test' \
--attach @/path/file-1.txt \
--attach @/path/file-2.txt
Resend an existing email
It can sometimes be useful to resend an existing email, such as from an .eml file. The raw data can be passed in via the --data option.
swaks \
--from [email protected] \
--to [email protected] \
--server in.smtp.sendamatic.net:587 \
--auth plain \
--tls \
--auth-user [mail credential user] \
--auth-password [mail credential password] \
--data @/path/file.eml
Send to multiple recipients
A comma seperated list of recipients can be passed into the --to option.
swaks \
--from [email protected] \
--to [email protected],[email protected] \
--server in.smtp.sendamatic.net:587 \
--auth plain \
--tls \
--auth-user [mail credential user] \
--auth-password [mail credential password]
Send a BCC email
A BCC email has envelope senders that doesn't appear in the 'To' header. Swaks gives you full control over both of these.
swaks \
--from [email protected] \
--to [email protected],[email protected] \
--header "To: [email protected]" \
--server in.smtp.sendamatic.net:587 \
--auth plain \
--tls \
--auth-user [mail credential user] \
--auth-password [mail credential password]
Adding headers
Headers can be added in a couple of ways. Say you wanted to add a List-ID header:
If the header already exists in DATA (e.g. you're resending an existing file), this first example will replace the header.
# Both these syntaxes behave in the same way
--header-List-ID "<mailing-list-name.domain.com>"
--header "List-ID: <mailing-list-name.domain.com>"
This second example will add a new header at the end of the existing headers, wether the same one already existed or not:
TLS
Slightly confusingly, the --tls flag refers to STARTTLS, which is a way of negotiating a secure connection over an unsecured port. If you wish to connect to a TLS secured port (typically port 465), use the --tls-on-connect flag instead.
swaks \
--from [email protected] \
--to [email protected] \
--server in.smtp.sendamatic.net:465 \
--auth plain \
--tls-on-connect \
--auth-user [mail credential user] \
--auth-password [mail credential password]