Subjectline.com
There's no point in creating amazing-looking emails if they aren't going to get opened.
The guys at Subjectline.com have a great tool to help you test your subject line, with a score out of 100 given based on a few variables such as urgency, the types of words you use and the length of your subject line.
Try it out here
Litmus' Gmail category tab test
Gmail categories are intended to make life easier by pre-sorting emails into a Primary, Social, Updates, Forums, or Promotions tab.
Depending on the type of email campaign you're sending, you can use this tool to test where your emails are likely to land.
Try it out here
Mailmodo's Dark Mode Checker
Different email clients render emails differently in dark mode.
Dark mode adoption is also on a high, which means you need to ensure that your emails are dark mode ready.
Use this tool to take a peek at what your emails are doing in dark mode in various inboxes.
Try it out here
Mailtester.com
No one wants their emails to go to spam. This basic tool is a great place to start to check if your emails are going to spam or not.
There are a ton of great tools out there, and many that give far more detail, but this tool is a great, free resource for any email sender to begin with.
Try it out here
Parcel's mailto link generator
If you're looking to make the process of replying to your emails simpler for readers, here's where to start.
Generate mailto links with a subject, body, cc and bcc quickly and easily with this tool.
Try it out here