diff --git a/.env.template b/.env.template
index 0189cc7a96..7417d0ba85 100644
--- a/.env.template
+++ b/.env.template
@@ -525,6 +525,13 @@
 ## Only use this as a last resort if you are not able to use a valid certificate.
 # SMTP_ACCEPT_INVALID_HOSTNAMES=false
 
+## Accept additional root certs
+## Paths to PEM files, separated by semicolons
+# SMTP_ADDITIONAL_ROOT_CERTS=
+
+## Use system root certificate store for TLS host verification
+# SMTP_USE_SYSTEM_ROOT_CERTS=true
+
 ##########################
 ### Rocket settings ###
 ##########################
diff --git a/src/config.rs b/src/config.rs
index 489a229d6a..ebdb56e590 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -3,6 +3,7 @@ use std::process::exit;
 use std::sync::RwLock;
 
 use job_scheduler_ng::Schedule;
+use lettre::transport::smtp::client::Certificate;
 use once_cell::sync::Lazy;
 use reqwest::Url;
 
@@ -674,6 +675,10 @@ make_config! {
         smtp_accept_invalid_certs:     bool,   true,   def,     false;
         /// Accept Invalid Hostnames (Know the risks!) |> DANGEROUS: Allow invalid hostnames. This option introduces significant vulnerabilities to man-in-the-middle attacks!
         smtp_accept_invalid_hostnames: bool,   true,   def,     false;
+        /// Accept additional root certs |> Paths to PEM files, separated by semicolons
+        smtp_additional_root_certs:    String, true,   option;
+        /// Use system root certificate store for TLS host verification
+        smtp_use_system_root_certs:    bool,   true,   def,     true;
     },
 
     /// Email 2FA Settings
@@ -886,6 +891,11 @@ fn validate_config(cfg: &ConfigItems) -> Result<(), Error> {
         if cfg._enable_email_2fa && cfg.email_token_size < 6 {
             err!("`EMAIL_TOKEN_SIZE` has a minimum size of 6")
         }
+
+        if let Some(additional_root_cert_paths) = &cfg.smtp_additional_root_certs {
+            let certificates = load_certificates(additional_root_cert_paths.as_str())?;
+            *SMTP_ADDITIONAL_ROOT_CERTS.write().unwrap() = certificates;
+        }
     }
 
     if cfg._enable_email_2fa && !(cfg.smtp_host.is_some() || cfg.use_sendmail) {
@@ -1018,6 +1028,24 @@ fn generate_smtp_img_src(embed_images: bool, domain: &str) -> String {
     }
 }
 
+pub static SMTP_ADDITIONAL_ROOT_CERTS: RwLock<Vec<Certificate>> = RwLock::new(Vec::new());
+
+fn load_certificates(smtp_additional_root_certs: &str) -> Result<Vec<Certificate>, Error> {
+    smtp_additional_root_certs
+        .split(';')
+        .filter(|path| !path.is_empty())
+        .map(|path| {
+            let cert = match std::fs::read(path) {
+                Ok(cert) => cert,
+                Err(e) => {
+                    err!(format!("Error loading additional SMTP root certificate file {path}.\n{e}"))
+                }
+            };
+            Certificate::from_pem(&cert).or_else(|e| err!(format!("Error decoding certificate file {path}.\n{e}")))
+        })
+        .collect()
+}
+
 /// Generate the correct URL for the icon service.
 /// This will be used within icons.rs to call the external icon service.
 fn generate_icon_service_url(icon_service: &str) -> String {
diff --git a/src/mail.rs b/src/mail.rs
index 151554a1fd..7d960c1ad4 100644
--- a/src/mail.rs
+++ b/src/mail.rs
@@ -6,7 +6,7 @@ use percent_encoding::{percent_encode, NON_ALPHANUMERIC};
 use lettre::{
     message::{Attachment, Body, Mailbox, Message, MultiPart, SinglePart},
     transport::smtp::authentication::{Credentials, Mechanism as SmtpAuthMechanism},
-    transport::smtp::client::{Tls, TlsParameters},
+    transport::smtp::client::{CertificateStore, Tls, TlsParameters},
     transport::smtp::extension::ClientId,
     Address, AsyncSendmailTransport, AsyncSmtpTransport, AsyncTransport, Tokio1Executor,
 };
@@ -17,6 +17,7 @@ use crate::{
         encode_jwt, generate_delete_claims, generate_emergency_access_invite_claims, generate_invite_claims,
         generate_verify_email_claims,
     },
+    config::SMTP_ADDITIONAL_ROOT_CERTS,
     error::Error,
     CONFIG,
 };
@@ -46,6 +47,12 @@ fn smtp_transport() -> AsyncSmtpTransport<Tokio1Executor> {
         if CONFIG.smtp_accept_invalid_certs() {
             tls_parameters = tls_parameters.dangerous_accept_invalid_certs(true);
         }
+        for cert in &*SMTP_ADDITIONAL_ROOT_CERTS.read().unwrap() {
+            tls_parameters = tls_parameters.add_root_certificate(cert.clone());
+        }
+        if !CONFIG.smtp_use_system_root_certs() {
+            tls_parameters = tls_parameters.certificate_store(CertificateStore::None);
+        }
         let tls_parameters = tls_parameters.build().unwrap();
 
         if CONFIG.smtp_security() == *"force_tls" {