First, mailto links violate the basic assumption on the Web that clicking a link will cause a new page to be displayed. Instead they surprise and annoy the user by dumping them into a different application. Second, the from -address inserted by the email program may not be the one where the user wants to receive the mailings. Send the user a confirmation message and don't activate the subscription unless you receive a positive reply. Yes, plain text can be a user interface.
Users change jobs and their old email address is taken over by the new employee.
This person never asked for the mail but will keep getting it. Well-meaning users will add an internal distribution list as a subscriber because they think that everybody in a certain department ought to get the messages. Some of these people will inevitably be annoyed about getting mail they didn't ask for. Unfortunately there is no way to protect against this problem since you cannot tell which email addresses refer to individual users and which are distribution lists. My preferred solution to the last two problems is to have the footer of each message state the email address to which the messages are sent.