See the //email/testTo: ("TestReceiver") Full Documentation for complete information.
Type in the email address of someone that you send email to.
Leave the Output Level set to "Detail".
Click the Run Test button.
When you click Run Test, //email/testTo: ("TestReceiver") performs all the steps that Internet email systems go through to send email. It records every command and byte of data it sends and every answer and byte of data that the other email system sends. TestReceiver never actually sends an email, it just gets as close as possible, learning as much about the remote system as it can.
Because CheckTLS focuses on security, TestReceiver tries to establish a secure (TLS) connection with the recipient's system. Along with recording everything, it looks at the security of the recipient's system for things like: certificate contents and signers, encryption algorithms, key lengths, hostname mis-matches, incorrect wild-card usage, weak cyphers, etc.
Confidence Factor
For all Output Levels, the first thing TestReceiver shows is our unique Confidence Factor. This is our "grade" (zero to 100) for the recipient's email system. It takes into account all the security information gathered while it was connected with the recipient's email system. For domains with multiple email servers (MX hosts), it weighs how many there are and their priority. It computes a single number for the given email address that is our opinion on how securely it will receive email.
We suggest that a Confidence Factor of 90 or above indicates that the email address is "secure".
MX Matrix
The next level of output is the MX Matrix. TestReceiver groups the steps of sending an email into 8 stages. The MX Matrix shows, for each MX host, how long each stage took and whether it was successful or not. Use the MX Matrix to look deeper into an email system, both down the matrix (by MX Host) and across the matrix (by stage), to show where strengths and weaknesses are in the system.
See the TestReceiver Full Documentation for more information about the MX Matrix stages.
Detail
The next levels of output are all Detail. Detail is the log of TestReceiver's interaction with the recipient's email system. Depending on the Output Level chosen it also shows what is inside the remote system's SSL Certificates and the details of the SSL connection established with the remote system.
See the TestReceiver Full Documentation for more information about what the Detail levels show.
The FULL Version has these additional input fields:
'SSLv23' uses a handshake compatible with SSL2.0, SSL3.0 and TLS1.x, while 'SSLv2', 'SSLv3', 'TLSv1', 'TLSv1_1' or 'TLSv1_2' restrict handshake and protocol to the specified version. All values are case-insensitive. Instead of 'TLSv1_1' and 'TLSv1_2' one can also use 'TLSv11' and 'TLSv12'. Support for 'TLSv1_1' and 'TLSv1_2' requires recent versions of Net::SSLeay and openssl.
Independent from the handshake format you can limit to set of accepted SSL versions by adding !version separated by ':'.
The CheckTLS default SSL Version is 'SSLv23' which allows any handshake version for testing purposes. CheckTLS issues a warning if the handshake negotiated is SSL2.0 and SSL3.0 which have serious security issues and should not be used anymore.
Most production systems use the default SSL Version 'SSLv23:!SSLv3:!SSLv2' which means that the handshake format is compatible to SSL2.0 and higher, but that the successful handshake is limited to TLS1.0 and higher, that is no SSL2.0 or SSL3.0 because both of these versions have serious security issues and should not be used anymore. You can also use !TLSv1_1 and !TLSv1_2 to disable TLS versions 1.1 and 1.2 while still allowing TLS version 1.0.
Setting the version instead to 'TLSv1' might break interaction with older clients, which need and SSL2.0 compatible handshake. On the other side some clients just close the connection when they receive a TLS version 1.1 request. In this case setting the version to 'SSLv23:!SSLv2:!SSLv3:!TLSv1_1:!TLSv1_2' might help.
If this option is set the cipher list for the connection will be set to the given value, e.g. something like 'ALL:!LOW:!EXP:!aNULL'. Look into the OpenSSL documentation for more details.
Unless you fail to contact your peer because of no shared ciphers it is recommended to leave this option at the default setting. The default setting prefers ciphers with forward secrecy, disables anonymous authentication and disables known insecure ciphers like MD5, DES etc. This gives a grade A result at the tests of SSL Labs. To use the less secure OpenSSL builtin default (whatever this is) set SSL_cipher_list to ''.
The following options are restricted. They can only be used on systems that you directly control and that will not report CheckTLS as a threat. Improper use will harm CheckTLS.com and we will block your access and cancel your subscription without refund.
These options are not useful for testing the security of an email server. They do not affect the Confidence Factor and have no bearing on the security of emails.
Send an SMTP "RCPT TO" command.
Note that this option can be seen as a "sender callout" (See Calllback Verification). Sender callouts are a controversial subject with strong opinions both ways. (Google "sender callout" to see various opinions.)
Actually send a test email message.
Note that this will send one email per MX, which on a large email system could be many emails to the same address. Use either the Quick or the eMail MX Host options above to target just one MX host.
See what else you can test: How To.