Fix clippy warnings

lettre/src/lib.rs

@@ -50,7 +50,7 @@ pub struct EmailAddress(String);
 impl EmailAddress {
     pub fn new(address: String) -> EmailResult<EmailAddress> {
         if !EmailAddress::is_valid(&address) {
-            Err(Error::InvalidEmailAddress)?;
+            return Err(Error::InvalidEmailAddress);
         }
         Ok(EmailAddress(address))
     }
@@ -108,7 +108,7 @@ impl Envelope {
     /// Creates a new envelope, which may fail if `to` is empty.
     pub fn new(from: Option<EmailAddress>, to: Vec<EmailAddress>) -> EmailResult<Envelope> {
         if to.is_empty() {
-            Err(Error::MissingTo)?;
+            return Err(Error::MissingTo);
         }
         Ok(Envelope {
             forward_path: to,

lettre/src/smtp/commands.rs

@@ -20,6 +20,7 @@ pub struct EhloCommand {
 
 impl Display for EhloCommand {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        #[allow(clippy::write_with_newline)]
         write!(f, "EHLO {}\r\n", self.client_id)
     }
 }
@@ -165,7 +166,7 @@ pub struct VrfyCommand {
 
 impl Display for VrfyCommand {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        #[cfg_attr(feature = "cargo-clippy", allow(clippy::write_with_newline))]
+        #[allow(clippy::write_with_newline)]
         write!(f, "VRFY {}\r\n", self.argument)
     }
 }
@@ -186,6 +187,7 @@ pub struct ExpnCommand {
 
 impl Display for ExpnCommand {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        #[allow(clippy::write_with_newline)]
         write!(f, "EXPN {}\r\n", self.argument)
     }
 }

lettre_email/src/lib.rs

@@ -439,7 +439,7 @@ impl EmailBuilder {
                 .message
                 .header(Header::new_with_value("From".into(), from).unwrap());
         } else {
-            Err(Error::Envelope(LettreError::MissingFrom))?;
+            return Err(Error::Envelope(LettreError::MissingFrom));
         }
         if !self.cc.is_empty() {
             self.message = self