use time::SystemTime instead of time::Instant as the latter may use libc::clock_gettime(CLOCK_MONOTONIC) eg. on android and does not advance while being in deep sleep mode. therefore, time::Instant is not a reliable way for timeouts or stoping times.

src/smtp/send.rs

@@ -53,7 +53,7 @@ impl Smtp {
                 "Message len={} was smtp-sent to {}",
                 message_len, recipients_display
             )));
-            self.last_success = Some(std::time::Instant::now());
+            self.last_success = Some(std::time::SystemTime::now());
 
             Ok(())
         } else {