Affixa is a desktop application; after installation, it will ask to become your default e-mail client, which is basically required for it to work. Its main feature is taking over "mailto" links and adding the possibility to send stuff directly to your webmail accounts, such as Gmail and Yahoo Mail. It also has the option of automatically employing drop.io for sending large files; automatic shrinking of attachments (for example, you can set all images you send to be shrunk to several predefined sizes), and you can even set advanced rules for certain attachments, if you think they might cause problems for certain recepients.
Whether you're going to like Affixa or not depends a lot on your e-mailing habits. I, for example, am so used to merely opening Gmail and doing everything e-mail related from there, that most of Affixa's features seem superfluous.
If your job requires you to spend several hours checking and sending e-mails, and you're looking to make better use of that time, you should definitely check what Affixa has to offer.
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