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The **Simple Mail Transfer Protocol (SMTP)** is a protocol utilized within the TCP/IP suite for the **sending and receiving of e-mail**. Due to its limitations in queuing messages at the recipient's end, SMTP is often employed alongside either **POP3 or IMAP**. These additional protocols enable users to store messages on a server mailbox and to periodically download them.
In practice, it is common for **e-mail programs** to employ **SMTP for sending e-mails**, while utilizing **POP3 or IMAP for receiving** them. On systems based on Unix, **sendmail** stands out as the SMTP server most frequently used for e-mail purposes. The commercial package known as Sendmail encompasses a POP3 server. Furthermore, **Microsoft Exchange** provides an SMTP server and offers the option to include POP3 support.
If you have the opportunity to **make the victim send you a email** (via contact form of the web page for example), do it because **you could learn about the internal topology** of the victim seeing the headers of the mail.
You can also get an email from a SMTP server trying to **send to that server an email to a non-existent address** (because the server will send to the attacker a NDN mail). But, be sure that you send the email from an allowed address (check the SPF policy) and that you can receive NDN messages.
You should also try to **send different contents because you can find more interesting information** on the headers like: `X-Virus-Scanned: by av.domain.com`\
If the server supports NTLM auth (Windows) you can obtain sensitive info (versions). More info [**here**](https://medium.com/@m8r0wn/internal-information-disclosure-using-hidden-ntlm-authentication-18de17675666).
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**Delivery Status Notification Reports**: If you send an **email** to an organisation to an **invalid address**, the organisation will notify that the address was invalided sending a **mail back to you**. **Headers** of the returned email will **contain** possible **sensitive information** (like IP address of the mail services that interacted with the reports or anti-virus software info).
Organizations are prevented from having unauthorized email sent on their behalf by employing **SPF**, **DKIM**, and **DMARC** due to the ease of spoofing SMTP messages.
A **complete guide to these countermeasures** is made available at [https://seanthegeek.net/459/demystifying-dmarc/](https://seanthegeek.net/459/demystifying-dmarc/).
SPF [was "deprecated" in 2014](https://aws.amazon.com/premiumsupport/knowledge-center/route53-spf-record/). This means that instead of creating a **TXT record** in `_spf.domain.com` you create it in `domain.com` using the **same syntax**.\
Moreover, to reuse previous spf records it's quiet common to find something like `"v=spf1 include:_spf.google.com ~all"`
**Sender Policy Framework** (SPF) is a mechanism that enables Mail Transfer Agents (MTAs) to verify whether a host sending an email is authorized by querying a list of authorized mail servers defined by the organizations. This list, which specifies IP addresses/ranges, domains, and other entities **authorized to send email on behalf of a domain name**, includes various "**Mechanisms**" in the SPF record.
| ALL | Matches always; used for a default result like `-all` for all IPs not matched by prior mechanisms. |
| A | If the domain name has an address record (A or AAAA) that can be resolved to the sender's address, it will match. |
| IP4 | If the sender is in a given IPv4 address range, match. |
| IP6 | If the sender is in a given IPv6 address range, match. |
| MX | If the domain name has an MX record resolving to the sender's address, it will match (i.e. the mail comes from one of the domain's incoming mail servers). |
| PTR | If the domain name (PTR record) for the client's address is in the given domain and that domain name resolves to the client's address (forward-confirmed reverse DNS), match. This mechanism is discouraged and should be avoided, if possible. |
| EXISTS | If the given domain name resolves to any address, match (no matter the address it resolves to). This is rarely used. Along with the SPF macro language it offers more complex matches like DNSBL-queries. |
| INCLUDE | References the policy of another domain. If that domain's policy passes, this mechanism passes. However, if the included policy fails, processing continues. To fully delegate to another domain's policy, the redirect extension must be used. |
| REDIRECT | <p>A redirect is a pointer to another domain name that hosts an SPF policy, it allows for multiple domains to share the same SPF policy. It is useful when working with a large amount of domains that share the same email infrastructure.</p><p>It SPF policy of the domain indicated in the redirect Mechanism will be used.</p> |
It's also possible to identify **Qualifiers** that indicates **what should be done if a mechanism is matched**. By default, the **qualifier "+"** is used (so if any mechanism is matched, that means it's allowed).\
You usually will note **at the end of each SPF policy** something like: **\~all** or **-all**. This is used to indicate that **if the sender doesn't match any SPF policy, you should tag the email as untrusted (\~) or reject (-) the email.**
* **`+`**: Corresponds to a PASS result. By default, mechanisms assume this qualifier, making `+mx` equivalent to `mx`.
* **`?`**: Represents a NEUTRAL result, treated similarly to NONE (no specific policy).
* **`~`**: Denotes SOFTFAIL, serving as a middle ground between NEUTRAL and FAIL. Emails meeting this result are typically accepted but marked accordingly.
* **`-`**: Indicates FAIL, suggesting that the email should be outright rejected.
In the upcoming example, the **SPF policy of google.com** is illustrated. Note the inclusion of SPF policies from different domains within the first SPF policy:
Traditionally it was possible to spoof any domain name that didn't have a correct/any SPF record. **Nowadays**, if **email** comes from a **domain without a valid SPF record** is probably going to be **rejected/marked as untrusted automatically**.
DKIM is utilized to sign outbound emails, allowing their validation by external Mail Transfer Agents (MTAs) through the retrieval of the domain's public key from DNS. This public key is located in a domain's TXT record. To access this key, one must know both the selector and the domain name.
For instance, to request the key, the domain name and selector are essential. These can be found in the mail header `DKIM-Signature`, e.g., `d=gmail.com;s=20120113`.
A command to fetch this information might look like:
DMARC enhances email security by building on SPF and DKIM protocols. It outlines policies that guide mail servers in the handling of emails from a specific domain, including how to deal with authentication failures and where to send reports about email processing actions.
The following was originally posted on openspf.org, which used to be a great resource for this kind of thing.
> The Demon Question: What about subdomains?
>
> If I get mail from pielovers.demon.co.uk, and there's no SPF data for pielovers, should I go back one level and test SPF for demon.co.uk? No. Each subdomain at Demon is a different customer, and each customer might have their own policy. It wouldn't make sense for Demon's policy to apply to all its customers by default; if Demon wants to do that, it can set up SPF records for each subdomain.
>
> So the advice to SPF publishers is this: you should add an SPF record for each subdomain or hostname that has an A or MX record.
>
> Sites with wildcard A or MX records should also have a wildcard SPF record, of the form: \* IN TXT "v=spf1 -all"
This makes sense - a subdomain may very well be in a different geographical location and have a very different SPF definition.
When emails are sent, ensuring they don't get flagged as spam is crucial. This is often achieved through the use of a **relay server that is trusted by the recipient**. However, a common challenge is that administrators might not be fully aware of which **IP ranges are safe to allow**. This lack of understanding can lead to mistakes in setting up the SMTP server, a risk frequently identified in security assessments.
A workaround that some administrators use to avoid email delivery issues, especially concerning communications with potential or ongoing clients, is to **allow connections from any IP address**. This is done by configuring the SMTP server's `mynetworks` parameter to accept all IP addresses, as shown below:
For checking whether a mail server is an open relay (which means it could forward email from any external source), the `nmap` tool is commonly used. It includes a specific script designed to test this. The command to conduct a verbose scan on a server (for example, with IP 10.10.10.10) on port 25 using `nmap` is:
## The receiver won't be able to check it, but the email will appear as signed (and therefore, more trusted)
dkim_selector="s1"
sig = dkim.sign(message=msg_data,selector=str(dkim_selector).encode(),domain=sender_domain.encode(),privkey=dkim_private_key.encode(),include_headers=headers)
**Find more information about these protections in** [**https://seanthegeek.net/459/demystifying-dmarc/**](https://seanthegeek.net/459/demystifying-dmarc/)
Usually, if installed, in `/etc/postfix/master.cf` contains **scripts to execute** when for example a new mail is receipted by a user. For example the line `flags=Rq user=mark argv=/etc/postfix/filtering-f ${sender} -- ${recipient}` means that `/etc/postfix/filtering` will be executed if a new mail is received by the user mark.
SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used in sending and receiving e-mail. However, since it is limited in its ability to queue messages at the receiving end, it is usually used with one of two other protocols, POP3 or IMAP, that let the user save messages in a server mailbox and download them periodically from the server.
**Instantly available setup for vulnerability assessment & penetration testing**. Run a full pentest from anywhere with 20+ tools & features that go from recon to reporting. We don't replace pentesters - we develop custom tools, detection & exploitation modules to give them back some time to dig deeper, pop shells, and have fun.
<summary><strong>Learn AWS hacking from zero to hero with</strong><ahref="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>
* If you want to see your **company advertised in HackTricks** or **download HackTricks in PDF** Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** us on **Twitter** 🐦 [**@carlospolopm**](https://twitter.com/hacktricks_live)**.**
* **Share your hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
