Cannot send mail with attachments

Read the article : Mail Drop limits - Apple Support

Mail Drop lets you send large files like videos, presentations, and images through iCloud. If you shared links through Mail Drop that are no longer available, you might have exceeded one or more of the service limits.

With Mail Drop, you can send attachments up to 5 GB in size. You can send these attachments right from Mail on your Mac, the Mail app on your iPhone, iPad, or iPod touch, and from iCloud.com on your Mac or PC. All files types are supported and attachments don’t count against your iCloud storage. If a message, including its attachments, is larger than your Internet Service Provider (ISP) limit, Mail will ask you to send the attachments using Mail Drop.

Attachments might not be sent using Mail Drop if they exceed these limits:

• The message, including its attachments, is larger than 5 GB. Try sending the contents of your message in multiple email messages or compressing your attachments to make them as small as possible.
• An uncompressed folder is attached to the message. Use these steps to compress your attachments.
• Too many messages have been sent or the maximum number of recipients has been exceeded. Learn about mailbox size and message sending limits.
• You reached the 1 TB Mail Drop storage limit. Each attachment expires after 30 days, so you can send new attachments after earlier attachments expire and more storage becomes available.

You should also review these limits:

• Mail Drop works with a Mac using OS X Yosemite or later, an iPhone, iPad, or iPod touch with iOS 9.2, and a Mac or PC with an updated browser. See the system requirements for Mail Drop.
• After you send an email, the recipients have 30 days to download the Mail Drop attachments.
• The recipient’s email app might have a smaller message size limit that prevents the attachment from being delivered.
• The time it takes to upload or download an attachment can vary depending on the speed of your Internet connection and the size of your attachment.
• The recipients might not be able to access your attachment if the link has an excessive amount of downloads or high traffic.

And a second article : Mail for Mac: Add attachments in Mail

Ok, so we know that your company-provided Mac is running macOS Sierra and that you are required to use its Mail app, but we will need to know who is providing the email account that you are trying to send an email attachment with? If it is your company's email server, do they have any types of security restrictions preventing certain types of attachments to be sent or received?

For example, it is very common for companies to restrict PDF or EXE files as attachments. If you are using a company email account, you may want to check with your IT department about that.