1. Ambiguous attachment names
Don't confuse your recipients by sending attachments with ambiguous names.
Name your attachments properly so that your recipients know what the file contains before even opening it.
Additionally, naming your files properly reduces the risk of sending the wrong attachment.
2. Calling People Out
Calling someone out (shaming or criticizing someone harshly) is not encouraged in any situation especially in emails.
Those messages will be there for a very long time and the effects may not be reversible.
3. Not replying to the last email in the thread
Always reply to the latest email in a chain of emails.
Not doing so could confuse others and take the conversation somewhere else.
4. Wall Of Text
Walls of text are very long paragraphs with many sentences and having no breaks in between.
It is just poor writing and no one can read those.
Use smaller paragraphs with two to three shorts sentences at max.
5. Long Signatures
Very few things are more annoying than a signature that spans half the page especially when the content of the message is just one line.
Your signature needs to be concise, brief but comprehensive.
6. Marking every email as urgent
This gives the impression that your work is more important than others'.
People will read into that and start treating all of your urgent emails as normal, which will be an issue when something really urgent comes up.
7. Changing the message title in a thread
When the email that you are typing is part of a chain of emails or a thread, don't alter the subject.
Doing so will break the chain and start a new thread, which makes it hard for everyone to follow.
8. Mixing "TO" and "CC"
The recipients in the "TO" field are primarily the people that are expected to reply or take action.
"CC" is for recipients that are not expected to do anything but need to know what is going on.
Don't mix the two.
9. Don't use CAPS
Don't write complete sentences in CAPS. CAPS means that you are shouting and it is also very hard to read.
10. Replying to everyone
Before you reply to everyone, review the recipient list and include only the ones that need to be there.
Help others unclutter their mailboxes.
11. Confusing Subjects
People are in such a hurry to send their emails that they will type just one word or even just leave the subject field blank.
Write subjects that are descriptive enough and that can be easily searched.
12. Not using an out of office message
If you are away from the office, let people know so that they can act accordingly.
If you don't, others might assume that you have read the email and that you are addressing the matter at hand.
13. Typing email addresses before the message
I came to know about this one from a bad experience.
One time I typed all the addresses first and proceeded to type the message. After ten or so minutes I hit the send button only to find out later that my email contained sensitive information that one of the recipients (that I forgot that he was there) should not receive.
14. Using email for everything
Email is great but it is not the ideal communication medium for every situation.
Calling or meeting someone might more efficient in many cases.
15. Demanding reply after work hours
Please don't do that. Respect others' personal time if you expect others to respect yours.
16. Midnight emails
This is very common among employees who want to show others that they are hardworking and dedicated to their jobs.
Spare others that little beep sound on their phones in the middle of the night.
Everyone knows that emails can be scheduled to be sent at any time.