Add EML parsing functionality to mail package

Added two new functions `EMLToMsg` and `readEML` to the `mail` package. `EMLToMsg` function opens and parses a .eml file and returns a pre-filled Msg pointer. `readEML` opens an EML file and uses net/mail to parse the header and body. These changes are made to provide support for EML file parsing, which is a common requirement in many email-based applications.

eml.go

@@ -0,0 +1,97 @@
+package mail
+
+import (
+	"errors"
+	"fmt"
+	nm "net/mail"
+	"os"
+)
+
+// EMLToMsg will open an parse a .eml file at a provided file path and return a
+// pre-filled Msg pointer
+func EMLToMsg(fp string) (*Msg, error) {
+	m := &Msg{
+		addrHeader:    make(map[AddrHeader][]*nm.Address),
+		genHeader:     make(map[Header][]string),
+		preformHeader: make(map[Header]string),
+		mimever:       Mime10,
+	}
+
+	pm, err := readEML(fp)
+	if err != nil || pm == nil {
+		return m, fmt.Errorf("failed to parse EML file: %w", err)
+	}
+
+	if err := parseEMLHeaders(&pm.Header, m); err != nil {
+		return m, fmt.Errorf("failed to parse EML headers: %w", err)
+	}
+
+	return m, nil
+}
+
+// readEML opens an EML file and uses net/mail to parse the header and body
+func readEML(fp string) (*nm.Message, error) {
+	fh, err := os.Open(fp)
+	if err != nil {
+		return nil, fmt.Errorf("failed to open EML file: %w", err)
+	}
+	defer func() {
+		_ = fh.Close()
+	}()
+	pm, err := nm.ReadMessage(fh)
+	if err != nil {
+		return pm, fmt.Errorf("failed to parse EML: %w", err)
+	}
+	return pm, nil
+}
+
+func parseEMLHeaders(mh *nm.Header, m *Msg) error {
+	commonHeaders := []Header{
+		HeaderContentType, HeaderImportance, HeaderInReplyTo, HeaderListUnsubscribe,
+		HeaderListUnsubscribePost, HeaderMessageID, HeaderMIMEVersion, HeaderOrganization,
+		HeaderPrecedence, HeaderPriority, HeaderSubject, HeaderUserAgent, HeaderXMailer,
+		HeaderXMSMailPriority, HeaderXPriority,
+	}
+
+	// Extract address headers
+	if v := mh.Get(HeaderFrom.String()); v != "" {
+		if err := m.From(v); err != nil {
+			return fmt.Errorf(`failed to parse "From:" header: %w`, err)
+		}
+	}
+	if v := mh.Get(HeaderTo.String()); v != "" {
+		var als []string
+		pal, err := nm.ParseAddressList(v)
+		if err != nil {
+			return fmt.Errorf(`failed to parse address list: %w`, err)
+		}
+		for _, a := range pal {
+			als = append(als, a.String())
+		}
+		if err := m.To(als...); err != nil {
+			return fmt.Errorf(`failed to parse "To:" header: %w`, err)
+		}
+	}
+
+	// Extract date from message
+	d, err := mh.Date()
+	if err != nil {
+		switch {
+		case errors.Is(err, nm.ErrHeaderNotPresent):
+			m.SetDate()
+		default:
+			return fmt.Errorf("failed to parse EML date: %w", err)
+		}
+	}
+	if err == nil {
+		m.SetDateWithValue(d)
+	}
+
+	// Extract common headers
+	for _, h := range commonHeaders {
+		if v := mh.Get(h.String()); v != "" {
+			m.SetGenHeader(h, v)
+		}
+	}
+	return nil
+}