diff --git a/cli/src/main.rs b/cli/src/main.rs
index 6e00d92f1f..50283747eb 100644
--- a/cli/src/main.rs
+++ b/cli/src/main.rs
@@ -63,6 +63,11 @@ pub struct Cli {
 	#[clap(long, env, default_value = "false", parse(try_from_str))]
 	pub gmail_use_api: bool,
 
+	/// For Outlook/Office 365 email addresses, use Outlook's API instead of
+	/// connecting directly to their SMTP servers.
+	#[clap(long, env, default_value = "false", parse(try_from_str))]
+	pub outlook_use_api: bool,
+
 	/// Whether to check if a gravatar image is existing for the given email.
 	#[clap(long, env, default_value = "false", parse(try_from_str))]
 	pub check_gravatar: bool,
@@ -87,6 +92,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
 		.set_smtp_port(CONF.smtp_port)
 		.set_yahoo_use_api(CONF.yahoo_use_api)
 		.set_gmail_use_api(CONF.gmail_use_api)
+		.set_outlook_use_api(CONF.outlook_use_api)
 		.set_check_gravatar(CONF.check_gravatar);
 	if let Some(proxy_host) = &CONF.proxy_host {
 		input.set_proxy(CheckEmailInputProxy {
diff --git a/core/src/smtp/hotmail.rs b/core/src/smtp/hotmail.rs
index 2819738da2..d8c5a449e2 100644
--- a/core/src/smtp/hotmail.rs
+++ b/core/src/smtp/hotmail.rs
@@ -23,12 +23,15 @@ use fantoccini::{
 	ClientBuilder, Locator,
 };
 use futures::TryFutureExt;
+use reqwest::Error as ReqwestError;
 use serde::Serialize;
 use serde_json::Map;
 
 use super::SmtpDetails;
-use crate::util::ser_with_display::ser_with_display;
 use crate::LOG_TARGET;
+use crate::{
+	smtp::http_api::create_client, util::ser_with_display::ser_with_display, CheckEmailInput,
+};
 
 #[derive(Debug, Serialize)]
 pub enum HotmailError {
@@ -36,6 +39,8 @@ pub enum HotmailError {
 	Cmd(CmdError),
 	#[serde(serialize_with = "ser_with_display")]
 	NewSession(NewSessionError),
+	#[serde(serialize_with = "ser_with_display")]
+	ReqwestError(ReqwestError),
 }
 
 impl From<CmdError> for HotmailError {
@@ -50,6 +55,12 @@ impl From<NewSessionError> for HotmailError {
 	}
 }
 
+impl From<ReqwestError> for HotmailError {
+	fn from(error: ReqwestError) -> Self {
+		HotmailError::ReqwestError(error)
+	}
+}
+
 /// Check if a Hotmail/Outlook email exists by connecting to the password
 /// recovery page https://account.live.com/password/reset using a headless
 /// browser. Make sure you have a WebDriver server running locally before
@@ -140,9 +151,56 @@ pub async fn check_password_recovery(
 	})
 }
 
+/// Convert an email address to its corresponding OneDrive URL.
+fn get_onedrive_url(email_address: &str) -> String {
+	let (username, domain) = email_address
+		.split_once('@')
+		.expect("Email address syntax already validated.");
+	let (tenant, _) = domain
+		.split_once('.')
+		.expect("Email domain syntax already validated.");
+
+	format!(
+		"https://{}-my.sharepoint.com/personal/{}_{}/_layouts/15/onedrive.aspx",
+		tenant,
+		username.replace('.', "_"),
+		domain.replace('.', "_"),
+	)
+}
+
+/// Use HTTP request to verify if an Outlook email address exists.
+/// See: <https://www.trustedsec.com/blog/achieving-passive-user-enumeration-with-onedrive/>
+pub async fn check_outlook_api(
+	to_email: &EmailAddress,
+	input: &CheckEmailInput,
+) -> Result<SmtpDetails, HotmailError> {
+	let url = get_onedrive_url(to_email.as_ref());
+
+	let response = create_client(input, "outlook")?
+		.head(url)
+		.query(&[("email", to_email)])
+		.send()
+		.await?;
+
+	let email_exists = response.status() == 403;
+
+	log::debug!(
+		target: LOG_TARGET,
+		"[email={}] outlook response: {:?}",
+		to_email,
+		response
+	);
+
+	Ok(SmtpDetails {
+		can_connect_smtp: true,
+		is_deliverable: email_exists,
+		..Default::default()
+	})
+}
+
 #[cfg(test)]
 mod tests {
-	use super::check_password_recovery;
+	use super::{check_password_recovery, get_onedrive_url};
 	use async_smtp::EmailAddress;
 	use async_std::prelude::FutureExt;
 	use std::str::FromStr;
@@ -193,4 +251,12 @@ mod tests {
 		let f = f1.try_join(f2).await;
 		assert!(f.is_ok(), "{:?}", f);
 	}
+
+	#[test]
+	fn test_onedrive_url() {
+		let email_address = "lightmand@acmecomputercompany.com";
+		let expected = "https://acmecomputercompany-my.sharepoint.com/personal/lightmand_acmecomputercompany_com/_layouts/15/onedrive.aspx";
+
+		assert_eq!(expected, get_onedrive_url(email_address));
+	}
 }
diff --git a/core/src/smtp/mod.rs b/core/src/smtp/mod.rs
index 20d14f856d..be01c569b6 100644
--- a/core/src/smtp/mod.rs
+++ b/core/src/smtp/mod.rs
@@ -69,6 +69,11 @@ pub async fn check_smtp(
 			.await
 			.map_err(|err| err.into());
 	}
+	if input.outlook_use_api && host_lowercase.ends_with(".outlook.com.") {
+		return hotmail::check_outlook_api(to_email, input)
+			.await
+			.map_err(|err| err.into());
+	}
 	#[cfg(feature = "headless")]
 	if let Some(webdriver) = &input.hotmail_use_headless {
 		if host_lowercase.contains("outlook") {
diff --git a/core/src/util/input_output.rs b/core/src/util/input_output.rs
index 9e668e3db0..b894fb2e2d 100644
--- a/core/src/util/input_output.rs
+++ b/core/src/util/input_output.rs
@@ -95,6 +95,11 @@ pub struct CheckEmailInput {
 	///
 	/// Defaults to false.
 	pub gmail_use_api: bool,
+	/// For Outlook/Office 365 email addresses, use Outlook's API instead of
+	/// connecting directly to their SMTP servers.
+	///
+	/// Defaults to false.
+	pub outlook_use_api: bool,
 	// Whether to check if a gravatar image is existing for the given email.
 	//
 	// Defaults to false
@@ -132,6 +137,7 @@ impl Default for CheckEmailInput {
 			smtp_timeout: None,
 			yahoo_use_api: true,
 			gmail_use_api: false,
+			outlook_use_api: false,
 			check_gravatar: false,
 			retries: 2,
 		}
@@ -247,6 +253,13 @@ impl CheckEmailInput {
 		self
 	}
 
+	/// Set whether to use Outlook's API or connecting directly to their SMTP
+	/// servers. Defaults to false.
+	pub fn set_outlook_use_api(&mut self, use_api: bool) -> &mut CheckEmailInput {
+		self.outlook_use_api = use_api;
+		self
+	}
+
 	/// Whether to check if a gravatar image is existing for the given email.
 	/// Defaults to false.
 	pub fn set_check_gravatar(&mut self, check_gravatar: bool) -> &mut CheckEmailInput {