We've covered what to do with negative testimonials, but for best results you need to use the positive ones to their best effect. This doesn't mean peppering them throughout your email marketing campaign but accurately targeting them.
By all means have a collection of them on one page on your website to reassure those who might be concerned but do not leave it at that. Most will ignore them. In researching this article I found a page of feedback on a site I visit regularly that I'd never seen before.
The first thing to do is separate them into types. For instance, there will be those that praise customer relations, the robustness of your product, prompt delivery, after sales service and, hopefully, lots more particulars. It might be helpful to subdivide these into ones best for B2C and those for B2B.
Cross reference them if it suits your product. For instance, if you sell holidays, then destination, hotel, country, activity, full board and similar will allow you to identify the ones you want for a specific purpose.
If you know your audience you can pick which to use where. Those who have been on your email marketing list for some time will not need reassuring about customer service but might require a push towards a new product to replace the item you sold them some time ago. So a review which reinforces your message of great savings to be made is the one to go for.
Use a testimonial in the email you send on sign-up to your email marketing list. ‘Always so helpful’ is a great way to reassure them.
It is unlikely you will be able to use a testimonial effectively without editing it substantially. If you pull just a sentence from it, have the full one on your website and consider including a link to it. Cutting out repetition, that which confuses and comments of no interest to a potential customer is perfectly permissible as long as the sense of it remains. 'Thanks very much' cut from 'Well thanks very much for ruining my day' is not on. See the link below.
Avoid the banal unless you are struggling. 'I love your [product]' is not likely to convince anyone to part with money.
Remember that you cannot post a review which makes a claim that cannot be justified. This is explained in the CAP Guidance on https://www.cap.org.uk/News-reports/Media-Centre/2016/Insight-User-generated-content-is-king.aspx#.V0NYhZErKHs
This also covers editing.
You know that a reviewer's name is almost certainly 'personal data' and is covered by the DPA. Further, the review copy might well be copyright, so ensure your permission is well documented.
If you have a feedback page, then informing a contributor that by submitting the form they agree to it being published together with their name, that they have a right to have it removed, that you may edit the testimonial or publish it in part without changing the sense, can be useful but also send an email to tell them you are using it.
It can be useful, and polite, to thank the person for their kind words directly under the review, signing off with a name. I could well generate other readers to contribute.