You need to zip the folder and then attach the zip.

Right click on folder and select 'Send to' > 'compressed 'zipped' folder. Then attach the zipped folder.

Please note the size. Emails usually have a limit. Your server may have a higher limit than the server that receives the email. So check both limits. Whatever the size of your email, you need to be about 30% less than the limit as there are additional increases to email as base64 encoding adds up to 30% overhead.

TB needs a built-in ZIP function so a folder, or all separate attachments get zipped. And also split them to stay under the IP send limit. Trying to zip 6 files located in different places, then attach each, takes all day.

My zip utility will not split files into a designated size to stay below the IP send limit. Looking for another OSX zip utility.

re :TB needs a built-in ZIP function

TB is an email client. There are programs that zip files.

Please read some info here as it discusses methods on how to send large attachments. This might help your situation. https://support.mozilla.org/en-US/kb/filelink-large-attachments