One very common myth is that email marketing is dead because of social media. However, email offers direct communication with your audience. It helps with higher engagement rates, and better control over audience targeting.
The myth that more subscribers is better. I'd much prefer to have an engaged tight list vs a huge list that converts at a lower %.
Be comfortable with attrition.
"Hide that unsubscribe button & be scared of unsubscribes"
Narrowing down your audience and staying connected with users who share common interests with you often leads to better outcomes