When opening an email in a mobile device it will certainly have a different look than that of an email opened on a computer.
The main factor is the screen size, an email client (web or heavy) on a PC has a recommended width display limit for the signature object of 600px. When opening an email containing a width of 600px signature (big banners, big logo, big pictures, long horizontal text blocs, etc).
on mobile devices which screen supports 375px of width in portrait mode (for an iPhone X), the signature and images will be reinterpreted by the device and the email client that is used to try to fit the elements to size of the screen.
This may result in different image size being displayed, but also different spacing or alignment of elements as the phone tries to fit everything in the available physical space.
Other elements that get reinterpreted by the device itself or the email client used are :
Links (websites, emails, etc.)
and in some cases addresses (because of the numbers).
Mainly anything having a link might be underlined or turned blue.
Mobile phones tend to make phone numbers clickable in order to be able to launch a call.
All of these treatments vary from device to device and app to app and are all involuntary and out of the hands of Letsignit.
However, there are best practices that you could follow to prevent some of these impacts.
Creating signatures that are no longer than 300px. This way the elements' positions, alignments, spacings will most likely be respected.