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Does ggis Subscibe support XXX mailing list server?

At this time, it only supports;

1. ezmlm lists - subscribe by sending email to listname-subscribe-your=address.com@domain.com
2. mailman lists - subscribe by sending email to listname-request@domain.com

If you would like additional email server support, please submit a comment to that effect.

Can this be used to unsubscribe?

Yes, that is part of the full subscription management form.

How can I adjust the form's appearance

You should be able to fully control your form's appearance using CSS. The HTML output is fully embedded with CSS tags to make personalization easy. As an example:

input.ggis-subscribe-email { width:90%; }