If you are pulling the majority of your email signature details from Active Directory, the problem you might face is that some users don't have all the information entered into AD (generally because it doesn't relate to them - for example, mobile numbers aren't assigned to all employees, or they don't have access to one).
In this case, when an employee sends an email and a mobile number isn't entered into AD against their name, there is blank line in the signature when an email is sent.
Email signatures can be configured to exclude a certain field if information doesn't exist and essentially, stop the blank line from appearing:
Result when empty variable is removed:
How to remove 'Mobile:' including the line break:
The NULL syntax depends on your design, so you need to do this in your HTML source. Make sure you to preview before your save.