refactor: Use variables directly in formatted strings (#7284)

made with `cargo clippy --all --fix` then manually reviewed to ensure
this was the only thing that changed.

src/smtp.rs

@@ -242,7 +242,7 @@ pub(crate) async fn smtp_send(
                         // Yandex error "554 5.7.1 [2] Message rejected under suspicion of SPAM; https://ya.cc/..."
                         // should definitely go here, because user has to open the link to
                         // resume message sending.
-                        SendResult::Failure(format_err!("Permanent SMTP error: {}", err))
+                        SendResult::Failure(format_err!("Permanent SMTP error: {err}"))
                     }
                 }
                 async_smtp::error::Error::Transient(ref response) => {
@@ -471,7 +471,7 @@ pub(crate) async fn send_msg_to_smtp(
             }
             Ok(())
         }
-        SendResult::Failure(err) => Err(format_err!("{}", err)),
+        SendResult::Failure(err) => Err(format_err!("{err}")),
     }
 }
 
@@ -586,7 +586,7 @@ async fn send_mdn_rfc724_mid(
 
     let addr = contact.get_addr();
     let recipient = async_smtp::EmailAddress::new(addr.to_string())
-        .map_err(|err| format_err!("invalid recipient: {} {:?}", addr, err))?;
+        .map_err(|err| format_err!("invalid recipient: {addr} {err:?}"))?;
     let recipients = vec![recipient];
 
     match smtp_send(context, &recipients, &body, smtp, None).await {