diff --git a/auth.go b/auth.go
index 7888819..314cb50 100644
--- a/auth.go
+++ b/auth.go
@@ -4,7 +4,11 @@
 
 package mail
 
-import "errors"
+import (
+	"errors"
+	"fmt"
+	"strings"
+)
 
 // SMTPAuthType is a type wrapper for a string type. It represents the type of SMTP authentication
 // mechanism to be used.
@@ -134,3 +138,33 @@ var (
 	// authentication type.
 	ErrSCRAMSHA256PLUSAuthNotSupported = errors.New("server does not support SMTP AUTH type: SCRAM-SHA-256-PLUS")
 )
+
+// UnmarshalString satisfies the fig.StringUnmarshaler interface for the SMTPAuthType type
+// https://pkg.go.dev/github.com/kkyr/fig#StringUnmarshaler
+func (sa *SMTPAuthType) UnmarshalString(value string) error {
+	switch strings.ToLower(value) {
+	case "cram-md5", "crammd5", "cram":
+		*sa = SMTPAuthCramMD5
+	case "custom":
+		*sa = SMTPAuthCustom
+	case "login":
+		*sa = SMTPAuthLogin
+	case "none", "noauth", "no":
+		*sa = SMTPAuthNoAuth
+	case "plain":
+		*sa = SMTPAuthPlain
+	case "scram-sha-1", "scram-sha1", "scramsha1":
+		*sa = SMTPAuthSCRAMSHA1
+	case "scram-sha-1-plus", "scram-sha1-plus", "scramsha1plus":
+		*sa = SMTPAuthSCRAMSHA1PLUS
+	case "scram-sha-256", "scram-sha256", "scramsha256":
+		*sa = SMTPAuthSCRAMSHA256
+	case "scram-sha-256-plus", "scram-sha256-plus", "scramsha256plus":
+		*sa = SMTPAuthSCRAMSHA256PLUS
+	case "xoauth2", "oauth2":
+		*sa = SMTPAuthXOAUTH2
+	default:
+		return fmt.Errorf("unsupported SMTP auth type: %s", value)
+	}
+	return nil
+}
diff --git a/auth_test.go b/auth_test.go
new file mode 100644
index 0000000..984dbd6
--- /dev/null
+++ b/auth_test.go
@@ -0,0 +1,47 @@
+package mail
+
+import "testing"
+
+func TestSMTPAuthType_UnmarshalString(t *testing.T) {
+	tests := []struct {
+		name       string
+		authString string
+		expected   SMTPAuthType
+	}{
+		{"CRAM-MD5: cram-md5", "cram-md5", SMTPAuthCramMD5},
+		{"CRAM-MD5: crammd5", "crammd5", SMTPAuthCramMD5},
+		{"CRAM-MD5: cram", "cram", SMTPAuthCramMD5},
+		{"CUSTOM", "custom", SMTPAuthCustom},
+		{"LOGIN", "login", SMTPAuthLogin},
+		{"NONE: none", "none", SMTPAuthNoAuth},
+		{"NONE: noauth", "noauth", SMTPAuthNoAuth},
+		{"NONE: no", "no", SMTPAuthNoAuth},
+		{"PLAIN", "plain", SMTPAuthPlain},
+		{"SCRAM-SHA-1: scram-sha-1", "scram-sha-1", SMTPAuthSCRAMSHA1},
+		{"SCRAM-SHA-1: scram-sha1", "scram-sha1", SMTPAuthSCRAMSHA1},
+		{"SCRAM-SHA-1: scramsha1", "scramsha1", SMTPAuthSCRAMSHA1},
+		{"SCRAM-SHA-1-PLUS: scram-sha-1-plus", "scram-sha-1-plus", SMTPAuthSCRAMSHA1PLUS},
+		{"SCRAM-SHA-1-PLUS: scram-sha1-plus", "scram-sha1-plus", SMTPAuthSCRAMSHA1PLUS},
+		{"SCRAM-SHA-1-PLUS: scramsha1plus", "scramsha1plus", SMTPAuthSCRAMSHA1PLUS},
+		{"SCRAM-SHA-256: scram-sha-256", "scram-sha-256", SMTPAuthSCRAMSHA256},
+		{"SCRAM-SHA-256: scram-sha256", "scram-sha256", SMTPAuthSCRAMSHA256},
+		{"SCRAM-SHA-256: scramsha256", "scramsha256", SMTPAuthSCRAMSHA256},
+		{"SCRAM-SHA-256-PLUS: scram-sha-256-plus", "scram-sha-256-plus", SMTPAuthSCRAMSHA256PLUS},
+		{"SCRAM-SHA-256-PLUS: scram-sha256-plus", "scram-sha256-plus", SMTPAuthSCRAMSHA256PLUS},
+		{"SCRAM-SHA-256-PLUS: scramsha256plus", "scramsha256plus", SMTPAuthSCRAMSHA256PLUS},
+		{"XOAUTH2: xoauth2", "xoauth2", SMTPAuthXOAUTH2},
+		{"XOAUTH2: oauth2", "oauth2", SMTPAuthXOAUTH2},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var authType SMTPAuthType
+			if err := authType.UnmarshalString(tt.authString); err != nil {
+				t.Errorf("UnmarshalString() for type %s failed: %s", tt.authString, err)
+			}
+			if authType != tt.expected {
+				t.Errorf("UnmarshalString() for type %s failed: expected %s, got %s",
+					tt.authString, tt.expected, authType)
+			}
+		})
+	}
+}