More Options adds these input fields:

Show Test Progress

Show MX tests in real-time. A MX test is displayed live in the browser as it happens. All MXs are tested one after the other instead of at once, so we recommend using this option for just one specific MX.

Quick Test

Just do a quick "yes" or "no" test. This replaces the Confidence Factor with a Confidence QFactor, which may be a faster and better right-this-instant measure of security. It sets IGNORENOCONNECT=on, CHECKOCSP=off, MXPREFLIMIT=50%, MXHOSTLIMIT=1, STOPAFTER=EHLO2, TIMEOUT=11. These can then be overridden by setting them explicitly.

Check MTA-STS

Lookup and verify SMTP MTA Strict Transport Security (MTA-STS) and SMTP TLS Reporting settings. The largest email provider in the world enabled these in 2019: see About MTA-STS and TLS reporting - G Suite Admin Help.

Check DANE

Lookup and verify DNS-Based Authentication of Named Entities (DANE) Transport Layer Security (TLS) Protocol: TLSA settings. DANE allows X.509 digital certificates, commonly used for Transport Layer Security (TLS), to be bound to domain names using Domain Name System Security Extensions (DNSSEC).

Check Cert Sigs

Check which certificates signed which other certificates./dd>

Relax "*" match

Allow wild-card certs to match multiple levels of server name (see rfc-2818 section 3.1 paragraph 4).

SMTP TimeOut

How long (in seconds, default 30) to wait for the SMTP server to respond to a command. Use this if you are getting time-out errors on a slow connection or while testing a slow/busy server. While this allows you to test a slow system, needing more than 30 seconds indicates a problem and regular email will frequently fail.

MX Host

A specific MX host to test. Use this to focus on a single MX host, or when DNS does not return the right MX hosts. This can be a hostname (which will be DNS looked-up), or an IP address, or "name[ip]" which will connect(ip) but verify_hostname(name).

MX Port

The TCP port to use to talk to the email server, almost always 25 (SMTP) but can be 465 or 587. Leave blank to use 25.

MX Pref Limit

Limit the number of MX Preferences to test. Can be either a number or a percent (put % after digits). Leave blank to test all MX records. MX Host Limit (below) counts hosts, MX Entry Limit counts MXs (one MX Entry can have multiple Addresses), and MX Pref Limit counts MX Preferences (one preference can have multiple Entries).

MX Entry Limit

Limit the number of MX Entries (in preference order) to test. Can be either a number or a percent (put % after digits). Leave blank to test all MX records. MX Host Limit (next) counts hosts, MX Entry Limit counts MXs (one MX Entry can have multiple Addresses), and MX Pref Limit (above) counts MX Preferences (one preference can have multiple Entries).

MX Host Limit

Limit the number of MX Hosts to test. Can be either a number or a percent (put % after digits). Leave blank to test all hosts. MX Pref Limit (above) counts MX Preferences (one preference can have multiple Entries), MX Entry Limit (above) counts MX Entries (one MX Entry can have multiple Addresses), and MX Host Limit counts hosts. With IgnoreNoConnect (below), MX Host Limit will stop after the first N hosts respond (in any preference order).

Ignore No Connects

Remove all MX hosts that do not allow us to connect, as if they were not in the DNS MX lookup in the first place. Used to prevent hosts that are off-line from reducing the Confidence Factor.

Stop After

Stop the SMTP conversation after this step, one of (ANSWER,CONNECT,EHLO1,STARTTLS,EHLO2,MAILFROM,RCPTTO,DATA). Leave blank to run all.

IPv4

Test IPv4 MX hosts.

IPv6

Test IPv6 MX hosts. Currently our hosting company Digital Ocean blocks this.

No DNS Cache

Bypass all DNS caching. Starts by looking up the GTLD from the global root servers and follows the authority (SOA) chain up to the entered domain name.

Use this when changing or debugging DNS issues to see results immediately without having to wait for DNS entries to time out. (Obviously your changes will still have to timeout for the rest of the world.)

The test reports the server path it found from the root servers to the final SOA.

DNS Host

The DNS host to use for all lookups (MX, MTA-STS, DANE, etc). Use this to test how your email interacts with DNS entries, possibly before you publish new DNS.

Compel TLS

Try starting TLS even if server does not offer it, i.e. send a STARTTLS command even if server did not offer 250 STARTTLS.

Direct TLS

Start TLS immediately after connecting to server and before sending or receiving any commands or data (typically used with port 465).

Check CRL

Check if certificate is revoked on its Certificate Revocation List (CRL). Note: this can take several minutes and may time-out, but if you wait 10 minutes and try again it will work because we cache CRLs.

Check OCSP

Check if certificate is revoked by its Online Certificate Status Protocol (OCSP).

SSL Version

Sets the version(s) of the SSL protocol that can be used. From the OpenSSL documentation:

'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 ':' (colon).

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.

SSL Cipher List

A list of Ciphers that can be used. From the OpenSSL documentation:

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 ''.

CA Certs

A PEM encoded Certificate or Certificate Chain of trusted Certificate Authorities to use to determine if the server's certificate is properly signed. Use Show Our CA List to see the Chain used by CheckTLS.

SMTP Auth

To help minimize unauthorized use, some email systems require authentication (i.e. login/password) to access email. The "AUTH" fields allow you to connect to these systems. SMTP Auth specifies which AUTH mechanism to use (plain, login, CRAM-MD5, NTLM)

AUTH User

The userid for authentication.

AUTH Pass

The password for authentication.

Client Cert

The Client Certificate to send.

Client Key

The Client Certificate Key to send.

XSL URL

This adds the specified XSL stylesheet URL to the XML output. It is up to whatever system receives the XML to apply the stylesheet. See TestReceiver XSL for more information.

XSL to Run

This applys the specified XSL to the XML. The translated XML is output in place of the original. You can enter the XSL itself or a URL from which to retrieve the XSL. See TestReceiver XSL for more information.

SOCKS

Test from your IP address using SOCKS.
Format is [user[:pass]@]host:port, e.g.
   host:port
   user@host.port (SOCKS4)
   user:pass@host.port (SOCKS5)

SMTP Detail XML

For XML Output Formats only, turn this on to include the SMTP log as CDATA in the XML output. See the TestReceiver Full Documentation for more information.

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.

RCPT TO

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.)

Send Email

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.