After enabling 2FA, I can receive email, but can no longer send.
The SMTP server just hangs or rejects my connection.
Once you enable 2FA on your account, most SMTP application will need to use the same Application Passcode that you generated for the device and entered in incoming (IMAP connection). We do not recommend using a separate Application Passcode for SMTP, but you could generate one if you wish or if you do not remember the password.
If you are using an iOS or Android device, you might opt to configure it using Activesync, which will still require an application passcode, but you'll only need to enter it once.