Calling it a day...

.idea/workspace.xml

@@ -4,13 +4,12 @@
     <option name="autoReloadType" value="ALL" />
   </component>
   <component name="ChangeListManager">
-    <list default="true" id="b79e8e7a-d892-4ce4-8bf4-f9e45415b803" name="Changes" comment="Better context and connection handling">
+    <list default="true" id="b79e8e7a-d892-4ce4-8bf4-f9e45415b803" name="Changes" comment="Lots of cleanups and refactoring">
       <change beforePath="$PROJECT_DIR$/.idea/workspace.xml" beforeDir="false" afterPath="$PROJECT_DIR$/.idea/workspace.xml" afterDir="false" />
       <change beforePath="$PROJECT_DIR$/README.md" beforeDir="false" afterPath="$PROJECT_DIR$/README.md" afterDir="false" />
       <change beforePath="$PROJECT_DIR$/client.go" beforeDir="false" afterPath="$PROJECT_DIR$/client.go" afterDir="false" />
       <change beforePath="$PROJECT_DIR$/cmd/main.go" beforeDir="false" afterPath="$PROJECT_DIR$/cmd/main.go" afterDir="false" />
       <change beforePath="$PROJECT_DIR$/header.go" beforeDir="false" afterPath="$PROJECT_DIR$/header.go" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/mailmsg.go" beforeDir="false" afterPath="$PROJECT_DIR$/mailmsg.go" afterDir="false" />
     </list>
     <option name="SHOW_DIALOG" value="false" />
     <option name="HIGHLIGHT_CONFLICTS" value="true" />
@@ -97,7 +96,8 @@
     <MESSAGE value="Progress" />
     <MESSAGE value="Implemented SMTP AUTH" />
     <MESSAGE value="Better context and connection handling" />
-    <option name="LAST_COMMIT_MESSAGE" value="Better context and connection handling" />
+    <MESSAGE value="Lots of cleanups and refactoring" />
+    <option name="LAST_COMMIT_MESSAGE" value="Lots of cleanups and refactoring" />
   </component>
   <component name="VgoProject">
     <integration-enabled>true</integration-enabled>

README.md

@@ -16,6 +16,7 @@ mail and SMTP related tasks.
 Some of the features of this library:
 * [X] Only Standard Library dependant
 * [X] Modern, idiotmatic Go
+* [X] Sane and secure defaults
 * [X] SSL/TLS support
 * [X] StartTLS support with different policies
 * [X] Makes use of contexts for a better control flow and timeout/cancelation handling

client.go

@@ -12,11 +12,17 @@ import (
 	"time"
 )
 
-// DefaultPort is the default connection port cto the SMTP server
-const DefaultPort = 25
+// Defaults
+const (
+	// DefaultPort is the default connection port cto the SMTP server
+	DefaultPort = 25
 
-// DefaultTimeout is the default connection timeout
-const DefaultTimeout = time.Second * 30
+	// DefaultTimeout is the default connection timeout
+	DefaultTimeout = time.Second * 15
+
+	// DefaultTLSPolicy is the default STARTTLS policy
+	DefaultTLSPolicy = TLSMandatory
+)
 
 // Client is the SMTP client struct
 type Client struct {
@@ -84,8 +90,8 @@ func NewClient(h string, o ...Option) (*Client, error) {
 		cto:       DefaultTimeout,
 		host:      h,
 		port:      DefaultPort,
-		tlspolicy: TLSMandatory,
 		tlsconfig: &tls.Config{ServerName: h},
+		tlspolicy: DefaultTLSPolicy,
 	}
 
 	// Set default HELO/EHLO hostname
@@ -257,27 +263,8 @@ func (c *Client) DialWithContext(pc context.Context) error {
 		return err
 	}
 
-	if !c.ssl && c.tlspolicy != NoTLS {
-		est := false
-		st, _ := c.sc.Extension("STARTTLS")
-		if c.tlspolicy == TLSMandatory {
-			est = true
-			if !st {
-				return fmt.Errorf("STARTTLS mode set to: %q, but target host does not support STARTTLS",
-					c.tlspolicy)
-			}
-		}
-		if c.tlspolicy == TLSOpportunistic {
-			if st {
-				est = true
-			}
-		}
-		if est {
-			if err := c.sc.StartTLS(c.tlsconfig); err != nil {
-				return err
-			}
-		}
-		_, c.enc = c.sc.TLSConnectionState()
+	if err := c.tls(); err != nil {
+		return err
 	}
 
 	if err := c.auth(); err != nil {
@@ -333,6 +320,33 @@ func (c *Client) checkConn() error {
 	return nil
 }
 
+// tls tries to make sure that the STARTTLS requirements are satisfied
+func (c *Client) tls() error {
+	if !c.ssl && c.tlspolicy != NoTLS {
+		est := false
+		st, _ := c.sc.Extension("STARTTLS")
+		if c.tlspolicy == TLSMandatory {
+			est = true
+			if !st {
+				return fmt.Errorf("STARTTLS mode set to: %q, but target host does not support STARTTLS",
+					c.tlspolicy)
+			}
+		}
+		if c.tlspolicy == TLSOpportunistic {
+			if st {
+				est = true
+			}
+		}
+		if est {
+			if err := c.sc.StartTLS(c.tlsconfig); err != nil {
+				return err
+			}
+		}
+		_, c.enc = c.sc.TLSConnectionState()
+	}
+	return nil
+}
+
 // auth will try to perform SMTP AUTH if requested
 func (c *Client) auth() error {
 	if err := c.checkConn(); err != nil {

cmd/main.go

@@ -24,6 +24,7 @@ func main() {
 	m.ToIgnoreInvalid("test@test.de", "foo@bar.de", "blubb@blah.com")
 	m.CcIgnoreInvalid("cc@test.de", "bar.de", "cc@blah.com")
 	m.BccIgnoreInvalid("bcc@test.de", "bcc@blah.com")
+	m.SetHeader("Foo", "bar")
 	m.SetMessageID()
 	m.SetDate()
 	m.SetBulk()

header.go

@@ -8,6 +8,12 @@ type AddrHeader string
 
 // List of common generic header field names
 const (
+	// HeaderContentLang is the "Content-Language" header
+	HeaderContentLang Header = "Content-Language"
+
+	// HeaderContentType is the "Content-Type" header
+	HeaderContentType Header = "Content-Type"
+
 	// HeaderDate represents the "Date" field
 	// See: https://www.rfc-editor.org/rfc/rfc822#section-5.1
 	HeaderDate Header = "Date"
@@ -16,6 +22,10 @@ const (
 	// See: https://www.rfc-editor.org/rfc/rfc1036#section-2.1.5
 	HeaderMessageID Header = "Message-ID"
 
+	// HeaderMIMEVersion represents the "MIME-Version" field as per RFC 2045
+	// See: https://datatracker.ietf.org/doc/html/rfc2045#section-4
+	HeaderMIMEVersion Header = "MIME-Version"
+
 	// HeaderPrecedence is the "Precedence" genHeader field
 	HeaderPrecedence Header = "Precedence"