check if SENDMAIL_COMMAND is valid using 'which' crate

Cargo.lock

@@ -3230,6 +3230,7 @@ dependencies = [
  "url",
  "uuid",
  "webauthn-rs",
+ "which",
  "yubico",
 ]
 
@@ -3396,6 +3397,17 @@ dependencies = [
  "untrusted",
 ]
 
+[[package]]
+name = "which"
+version = "4.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2441c784c52b289a054b7201fc93253e288f094e2f4be9058343127c4226a269"
+dependencies = [
+ "either",
+ "libc",
+ "once_cell",
+]
+
 [[package]]
 name = "widestring"
 version = "0.5.1"

Cargo.toml

@@ -151,6 +151,7 @@ semver = "1.0.16"
 # Allow overriding the default memory allocator
 # Mainly used for the musl builds, since the default musl malloc is very slow
 mimalloc = { version = "0.1.34", features = ["secure"], default-features = false, optional = true }
+which = "4.4.0"
 
 # Strip debuginfo from the release builds
 # Also enable thin LTO for some optimizations

src/config.rs

@@ -749,11 +749,15 @@ fn validate_config(cfg: &ConfigItems) -> Result<(), Error> {
         }
 
         if cfg.use_sendmail {
-            if let Some(ref command) = cfg.sendmail_command {
+            let command = cfg.sendmail_command.as_deref().unwrap_or("sendmail");
-                let path = std::path::Path::new(&command);
+
+            let mut path = std::path::PathBuf::from(command);
 
             if !path.is_absolute() {
-                    err!(format!("path to sendmail command `{path:?}` is not absolute"));
+                match which::which(command) {
+                    Ok(result) => path = result,
+                    Err(_) => err!(format!("sendmail command {command:?} not found in $PATH")),
+                }
             }
 
             match path.metadata() {
@@ -777,7 +781,6 @@ fn validate_config(cfg: &ConfigItems) -> Result<(), Error> {
                     }
                 }
             }
-            }
         } else {
             if cfg.smtp_host.is_some() == cfg.smtp_from.is_empty() {
                 err!("Both `SMTP_HOST` and `SMTP_FROM` need to be set for email support without `USE_SENDMAIL`")