"unsubscribe" is typically a link within the email to a web site to process or just a reply with "unsubscribe" in the subject and this also needs to be processed.
what "volume" of email are you planning? there may be other tips for email marketing that need to be considered.
beverly is correct. There are other things to consider when it comes to email marketing. Volume is the first big question.
However the unsubscribes can be managed in FM using the PHP API or using the unsubscribe reply. Maybe there is another way as well. You should consider some of the email services out there in the world. They do a lot for a small fee.
Be careful with the whole concept of rolling-your-own because it will be easy earn the title of spammer. You could find your company blacklisted and then none of your email gets through. Not even regular old person-to-person email.
You might consider using a service like MailChimp. There are other advantages too one being you can get some statistics like read-rate.
Volume is the biggest issue...and it doesn't take much for your domain and IP address to be flagged as a spammer. We use Mandrill/MailChimp for our own use and for our clients since it integrates relatively easily with FileMaker.
I just thought I would share that 360Works CloudMail is a plugin for FileMaker which may be great for your use case. It leverages the power of Amazon Web Services' sending structure so you can send a large volume of emails, at a relatively low cost. It also features an Unsubscribe feature.
Wherever you put in [UNSUBSCRIBE] in the HTML, is where CloudMail will automatically replace it with the unsubscribe link. In order to make this look like a hyperlink in an email, simply reference it in your HTML by placing the [UNSUBSCRIBE] as the target link and include the text you would like to appear to your readers in a standard HTML link format. This link will direct your users to a static page to unsubscribe from your mailing list.
There are also a few other neat features for tracking and analyzing data from the emails that are sent. You can always try out CloudMail for free in demo mode; it runs for 2 hours every time you restart FileMaker. If you have any additional questions about it, feel free to email us at firstname.lastname@example.org
-Charis Booker, 360Works