Consequently, vertical mails never suffer from an opt-in problem: the user explicitly asked to hear more about a topic. Of course, you still need to quickly accommodate opt-out requests, but there's much less worry about irritating prospects.
In your CRM system, the vertical campaigns need to be visible to the sales and support team. They need to know what mails were sent to whom when, and whether they've been opened. If there are specific calls to action within the drip emails (e.g., "download this free plug-in"), these prospect actions need to be shown within the lead and contact record as well (typically as a series of activities whose names make an obvious linkage back to the email).
The vertical campaigns should also stop if there has been a phone or other in-person interaction between your company and the prospect. Part of the magic of drip marketing campaigns is maintaining the illusion of a person-to-person email sequence. You don't want the robot to actually show through.
To make all this work well, you're either going to have to buy one of the fancier marketing automation systems, or do a bunch of integration with your email blaster and website by writing a fair number of triggers for your CRM system.
The previous two use cases were focused on marketing and lead cultivation. But ongoing customer support is another huge use of email. And increasingly, these emails are being generated automatically. Again, there's no issue of opting in here. You know the email threads are relevant because they were started by a customer asking for help and are providing real time updates on their case.
There's also not a big problem of the robot showing through. It's now commonplace to have support-bots suggesting workarounds and possible solutions. That said, you need to avoid a cacophony of uncoordinated emails arriving at the customer's inbox. We typically recommend that your support team (and all of their bots) send all emails into the CRM system (using a feature like Salesforce.com's email-to-case functionality, or via custom code in other systems) so everyone can see the state of play from the customer's perspective.
There is a gray area here: when you're marketing a new service offering. Even though your customers may all find your offer relevant, solicitations must follow the rules of marketing emails, not support emails.
By keeping all conversations visible to all of your CRM users, you dramatically reduce the likelihood of duplicate or confused messages getting to your customers and prospects. This is one of the most important steps to highly effective CRM, and great customer interactions.