Summarize the following:
Some e-mail providers may not allow you to send ZIP files for reasons such as security concerns or incapability with ZIP files. Your recipient may not be able to open or extract ZIP files from their e-mail provider. Ask if they have a different e-mail that you could contact them with. If you are compressing a large number of files, the size of you ZIP file may become too large for your e-mail provider to send. Most e-mail providers will have limits to the sizes of your attachments. Consider creating two or more ZIP files and sending these files in separate e-mails. If you are sending an already compressed ZIP file, you will need to extract the files. Then compress them separately. Some e-mail providers may not allow ZIP files to be attached or only permit a limited list of file types that exclude ZIP files. You can change the extension of your ZIP file to trick your e-mail provider into thinking the file is not a ZIP file. The file should work exactly like a ZIP file but will just have a different extension name.  ZIP programs such as Winzip allow you to create custom file extensions. For example, if you type zea (zipped email attachment), the zipped attachments will be named "file.zea" rather than "file.zip". This would enable you to send the ZIP file without error. It's important to note that this method does not always work. As well, you may need to download third party programs to change the extension name of your ZIP file. Most e-mail providers will warn you if a file has not been completely uploaded if you attempt to send your e-mail. As well, some e-mail providers will display the upload progress for you to watch. When a file is uploaded, it should appear in the attachments section of your e-mail or just under the main body of the e-mail.

Summary:
Try a different e-mail provider. Separate large ZIP files. Use a custom file extension. Make sure your ZIP file attachment has completely uploaded before sending.