Windows has no sendmail, so let's exclude those tests for Windows

msg_nowin_test.go

@@ -0,0 +1,29 @@
+//go:build !windows
+
+package mail
+
+import "testing"
+
+// TestMsg_WriteToSendmailWithCommand tests the WriteToSendmailWithCommand() method of the Msg
+func TestMsg_WriteToSendmailWithCommand(t *testing.T) {
+	tests := []struct {
+		name string
+		sp   string
+		sf   bool
+	}{
+		{"Sendmail path: /dev/null", "/dev/null", true},
+		{"Sendmail path: /bin/cat", "/bin/cat", true},
+		{"Sendmail path: /is/invalid", "/is/invalid", true},
+		{"Sendmail path: /bin/echo", "/bin/echo", false},
+	}
+	m := NewMsg()
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			m.SetBodyString(TypeTextPlain, "Plain")
+			if err := m.WriteToSendmailWithCommand(tt.sp); err != nil && !tt.sf {
+				t.Errorf("WriteToSendmailWithCommand() failed: %s", err)
+			}
+			m.Reset()
+		})
+	}
+}

msg_test.go

@@ -1138,27 +1138,3 @@ func TestMsg_Write(t *testing.T) {
 		fmt.Printf("%s", wbuf.String())
 	}
 }
-
-// TestMsg_WriteToSendmailWithCommand tests the WriteToSendmailWithCommand() method of the Msg
-func TestMsg_WriteToSendmailWithCommand(t *testing.T) {
-	tests := []struct {
-		name string
-		sp   string
-		sf   bool
-	}{
-		{"Sendmail path: /dev/null", "/dev/null", true},
-		{"Sendmail path: /bin/cat", "/bin/cat", true},
-		{"Sendmail path: /is/invalid", "/is/invalid", true},
-		{"Sendmail path: /bin/echo", "/bin/echo", false},
-	}
-	m := NewMsg()
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			m.SetBodyString(TypeTextPlain, "Plain")
-			if err := m.WriteToSendmailWithCommand(tt.sp); err != nil && !tt.sf {
-				t.Errorf("WriteToSendmailWithCommand() failed: %s", err)
-			}
-			m.Reset()
-		})
-	}
-}