You can tell Enigmail to automatically encrypt outgoing emails whenever the public key of the recipient.
Just set "Encrypt messages by Default" in your account settings. Then in OpenPGP > Preferences > Key Selection select "No manual key selection" (you might first need to activate the option "Display expert settings"). This way, Messages are encrypted automatically if a public Key is available.
This method of default encryption works for "trusted" Public keys. So either you enable "Always trust UserID" (which is slightly insecure) or you have to check every public key and sign it (you should only sign them locally, non-exportable) to make the Key trustworthy.
There is one problem with this method: You never know if a mail will be encrypted when you just push the "Send Now" button. There is a workaround: "Send Later" and look in your "Unsent Messages" folder.
Thanks to Christian Marg for this tip!