You could have downloaded Mail Designer Pro 3.5 for Mac from our application library for free. The actual developer of this Mac application is equinux. The software is included in Productivity Tools.
From the developer:
Mail Designer Pro 2 lets you create HTML-mails and newsletters with an optimized layout for mobile devices. A mobile preview makes it easy to optimize your newsletter design for a variety of different mobile devices.
You can easily work on a single document with multiple colleagues. Lend the newsletter to the graphic designers to pep-up the graphics. After it's been returned, you can lend it to the editors to add the appropriate text.
You may want to check out more software for Mac, such as Mail Exporter Pro, Mail Designer 365 or Mail Extractor Pro, which might be related to Mail Designer Pro.
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