If you cannot send email through Mailbird, the issue is usually related to an antivirus program or personal firewall blocking SMTP access.
In order to test this, try disabling all antivirus programs and firewalls temporarily, while you try to send an email. If the email goes through, one of those programs is blocking the connection. Now try turning them back on one after the other, while you keep sending test emails, until an email no longer goes through. The program you turned on last was the offending one.
Now that you know which program was blocking access, you can usually set up an exclusion in it, to allow Mailbird access. Please see the program's website for details on how to do this, if the particular program is not mentioned below.
McAfee VirusScan Enterprise
For exception put in: mailbird.*