
How Gmail, Yahoo, and Apple email services affect views and clicks reporting in email campaigns by masking user data.
A user agent in the context of email refers to information about the operating system, browser, and device that recipients use to open their emails.
An email client is a software application, program or app that allows users to send, receive, and manage email messages. Examples include Microsoft Outlook, Gmail, Thunderbird, and Apple Mail.
An email open is captured with the inclusion of an invisible image in the email that works as a tracker. Therefore, first of all the recipient must allow image downloading when reading the email.
A proxy agent may trigger an open even before the recipient actually opens an email. But they do more than that.
Although the open is recorded, Google, Apple, Yahoo, and lately other ESPs will hide the real "User agent" and IP address thus not allowing us to determine the actual email client or the exact location of the recipient.
This is what is reported by Gmail:
Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.9.0.7) Gecko/2009021910 Firefox/3.0.7 (via ggpht.com GoogleImageProxy)
By Yahoo,
YahooMailProxy; https://help.yahoo.com/kb/yahoo-mail-proxy-SLN28749.html
By Apple when Mail Privacy Protection is active:
Mozilla/5.0
Yes, they do. Since the real IP address is replaced with a generic one, geolocation reports based on opens will not be 100% accurate. They still however provide a very good indication.
When a subscriber actually clicks a link in the newsletter the real IP address and user agent are captured. Geolocation reports based on clicks will be more accurate.
However, email proxies tend to follow all hyperlinks in a newsletter for security and spam detection purposes. This inflates the number of clicks reported and distorts your clicks reporting. nuevoMailer uses Safe Tracking Protection to minimize the impact of email proxies on click reporting.
No, they only affects users opening their email using the web interface and the mobile Gmail app. Users with desktop email clients (such as Outlook) and non-Gmail mobile apps are not affected.
For example, if you are using Outlook to open your Gmail the email client will appear as MS Outlook.
Due to the reasons mentioned above, you see Gmail and Yahoo Mail Proxy in your email clients report.
Mozilla/5.0 is reported also as a distinct category and this largely attributed to Apple's mail privacy protection (MPP). With the introduction of MPP the share of iPhone and iPad declined and was replaced by Mozilla/5.0.

IPs captured at opt-in forms are the real ones. With nuevoMailer you can do an advanced search, find and segment subscribers based on campaign clicks/opens by city or country and by opt-in city or country.
Marketers need to adjust their reliance on open rates and geolocation data derived from them.