Unfortunately, Mailbird does not have a native spam filter right now. Instead it uses the spam filter of your email provider. So if your email provider consider an email as a spam, so will Mailbird.
If you need to block an email address, you need to login to your webmail and block the email address in there. Then Mailbird will also block the email address.
I totally understand, that it's not the greatest method of handling spam. However, we plan to implement the native spam filter in the future.
However, watch this space: this year, we'll be making a public announcement about a brand new cross-platform application (currently under development). Early access will be exclusively provided to our customers.
Although we plan on providing routine updates on Mailbird 2.0 (our current product), only the cross-platform experience of Mailbird Next (i.e. for Mac, Windows, and mobile) will be available on our new platform (scheduled to be released this year). The "Block sender" feature will be included in the new product.
If you'd like to sign up for early access and an exclusive offer, please visit www.getmailbird.com and select "Mailbird for Mac" at the top of the page, and enter your email address to become a part of this new journey with Mailbird Next!