Allow explicitly defined smtp auth mechansim

.env.template

@@ -149,3 +149,4 @@
 # SMTP_SSL=true
 # SMTP_USERNAME=username
 # SMTP_PASSWORD=password
+# SMTP_AUTH_MECHANISM="Plain"

src/config.rs

@@ -345,6 +345,8 @@ make_config! {
         smtp_username:          String, true,   option;
         /// Password
         smtp_password:          Pass,   true,   option;
+        /// Json form auth mechanism |> Defaults for ssl is "Plain" and "Login" and nothing for non-ssl connections. Possible values: ["Plain", "Login", "Xoauth2"]
+        smtp_auth_mechanism:    String, true,   option;
     },
 }
 

src/mail.rs

@@ -1,4 +1,5 @@
 use lettre::smtp::authentication::Credentials;
+use lettre::smtp::authentication::Mechanism as SmtpAuthMechanism;
 use lettre::smtp::ConnectionReuseParameters;
 use lettre::{ClientSecurity, ClientTlsParameters, SmtpClient, SmtpTransport, Transport};
 use lettre_email::{EmailBuilder, MimeMultipartType, PartBuilder};
@@ -39,6 +40,17 @@ fn mailer() -> SmtpTransport {
         _ => smtp_client,
     };
 
+    let smtp_client = match &CONFIG.smtp_auth_mechanism() {
+        Some(auth_mechanism_json) => {
+            let auth_mechanism = serde_json::from_str::<SmtpAuthMechanism>(&auth_mechanism_json.clone());
+            match auth_mechanism {
+                Ok(auth_mechanism) => smtp_client.authentication_mechanism(auth_mechanism),
+                Err(_) => panic!("Failure to parse mechanism. Is it proper Json? Eg. `\"Plain\"` not `Plain`"),
+            }
+        },
+        _ => smtp_client,
+    };
+
     smtp_client
         .smtp_utf8(true)
         .connection_reuse(ConnectionReuseParameters::NoReuse)