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Send attachments to Windows users in Mail on Mac
Try these suggestions for sending email attachments to Windows users.
To send Windows-friendly attachments for all messages, choose Edit > Attachments, then select Always Send Windows-Friendly Attachments.
To send Windows-friendly attachments for a single message, click the Attachment button
in the toolbar of the new message window, then select Send Windows-Friendly Attachments (if the tickbox isn’t shown, click Options).
Send documents as PDFs instead of in their original format.
Use filename extensions (such as .docx for a Microsoft Word document).
If a Windows user receives two attachments (such as “MyFile” and “. _MyFile”), they can ignore the file with the underscore.