We all make mistakes. However, when you've been using email for a couple of decades, you tend to have perfected your craft. (I still tell people I'm a Professional Email Processor.) These are the mistakes more seasoned email users avoid at all costs.
1. Hit reply all
Few people who have been around tech for a while still make this mistake. You get into the habit of confirming who is getting your message. It's rare that you need to send a message back to all of the original recipients.
2. Forget to blind copy
Another thing technophiles rarely (if ever) do is forget to blind copy. It's a newbie mistake. You also get into the habit of confirming the recipients for every single email.
3. Send an email to the wrong person
There are a lot of people named John. There's only one person with my email address. That's why it's best not to just type a first name and search. If you have processed a lot of email, you know you have to always confirm the recipient's name. Twice.
4. Permanently delete a message
We're living in an age of almost unlimited and cheap storage. I have at least 2TB right now in my Gmail account, and I haven't deleted a single email in at least six to seven years (really). If you want to make an archive, just create another email address and keep all of the old messages.
5. Use Outlook on the desktop
It's a waste of time to use the desktop client. Plus, if you switch between computers constantly, it means you probably have messages downloaded to one of the systems and not the others. I don't ever use Outlook, because it's so time consuming and not as accessible.
6. Try to send large files as attachments
New email users get an "oops" moment when they send a file that's too big for the recipient. Eventually, you realize there is a much better way. For example, you can upload the file to Google Drive and then just send the link.
7. Forward without explanation
This is more of a best practice, but there is a tech angle here, too. When you forward a message without any explanation, it means the recipient has to figure out why you sent it. (Full disclosure: I sometimes forget this rule.) Add a quick comment--it's not like it takes up extra bandwidth.
8. Have tons of customizations in your signature line
You might not realize this, but not every recipient can see the blinking animated cursor next to your name. On my phone especially, I read most messages as basic text. It's best to use a simple signature you know anyone can read on any device.
9. Send an email to dozens of recipients
This is a mistake people make when they accidentally hit "select all" in their email program and hit send. If you have a legitimate email to send to many people, it's far better to use an app like MailChimp.
10. Spam people
Here's one last mistake new users make. They don't quite realize that there are laws against sending hundreds and hundreds of unsolicited emails to people who did not request the message.