#21: Add possbility to set dedicated envelope from address

- Added `EnvelopeFrom()` and `EnvelopeFromFormat()` methods analogous to the `From()` `FromFormat()` methods
- Changed MsgWriter logic for envelope from addresses
- Adjusted `Msg.GetSender()` to return the envelope from first and only mail body from if the envelope is not set

header.go

@@ -81,7 +81,7 @@ const (
 	HeaderXPriority Header = "X-Priority"
 )
 
-// List of common generic header field names
+// List of common address header field names
 const (
 	// HeaderBcc is the "Blind Carbon Copy" header field
 	HeaderBcc AddrHeader = "Bcc"
@@ -89,6 +89,10 @@ const (
 	// HeaderCc is the "Carbon Copy" header field
 	HeaderCc AddrHeader = "Cc"
 
+	// HeaderEnvelopeFrom is the envelope FROM header field
+	// It's not included in the mail body but only used by the Client for the envelope
+	HeaderEnvelopeFrom AddrHeader = "EnvelopeFrom"
+
 	// HeaderFrom is the "From" header field
 	HeaderFrom AddrHeader = "From"
 

msg.go

@@ -197,6 +197,18 @@ func (m *Msg) SetAddrHeaderIgnoreInvalid(h AddrHeader, v ...string) {
 	m.addrHeader[h] = al
 }
 
+// EnvelopeFrom takes and validates a given mail address and sets it as envelope "FROM"
+// addrHeader of the Msg
+func (m *Msg) EnvelopeFrom(f string) error {
+	return m.SetAddrHeader(HeaderEnvelopeFrom, f)
+}
+
+// EnvelopeFromFormat takes a name and address, formats them RFC5322 compliant and stores them as
+// the envelope FROM address header field
+func (m *Msg) EnvelopeFromFormat(n, a string) error {
+	return m.SetAddrHeader(HeaderEnvelopeFrom, fmt.Sprintf(`"%s" <%s>`, n, a))
+}
+
 // From takes and validates a given mail address and sets it as "From" genHeader of the Msg
 func (m *Msg) From(f string) error {
 	return m.SetAddrHeader(HeaderFrom, f)
@@ -365,12 +377,16 @@ func (m *Msg) SetUserAgent(a string) {
 	m.SetHeader(HeaderXMailer, a)
 }
 
-// GetSender returns the currently set FROM address. If f is true, it will return the full
-// address string including the address name, if set
+// GetSender returns the currently set envelope FROM address. If no envelope FROM is set it will use
+// the first mail body FROM address. If ff is true, it will return the full address string including
+// the address name, if set
 func (m *Msg) GetSender(ff bool) (string, error) {
-	f, ok := m.addrHeader[HeaderFrom]
+	f, ok := m.addrHeader[HeaderEnvelopeFrom]
 	if !ok || len(f) == 0 {
-		return "", ErrNoFromAddress
+		f, ok = m.addrHeader[HeaderFrom]
+		if !ok || len(f) == 0 {
+			return "", ErrNoFromAddress
+		}
 	}
 	if ff {
 		return f[0].String(), nil

msgwriter.go

@@ -54,7 +54,22 @@ func (mw *msgWriter) writeMsg(m *Msg) {
 	m.addDefaultHeader()
 	m.checkUserAgent()
 	mw.writeGenHeader(m)
-	for _, t := range []AddrHeader{HeaderFrom, HeaderTo, HeaderCc} {
+
+	// Set the FROM header (or envelope FROM if FROM is empty)
+	hf := true
+	f, ok := m.addrHeader[HeaderFrom]
+	if !ok || len(f) == 0 {
+		f, ok = m.addrHeader[HeaderEnvelopeFrom]
+		if !ok || len(f) == 0 {
+			hf = false
+		}
+	}
+	if hf {
+		mw.writeHeader(Header(HeaderFrom), f[0].String())
+	}
+
+	// Set the rest of the address headers
+	for _, t := range []AddrHeader{HeaderTo, HeaderCc} {
 		if al, ok := m.addrHeader[t]; ok {
 			var v []string
 			for _, a := range al {