The current issue of IR Watch features this question. In a nutshell:
1. View the source code of an email and identify the time stamps from any two mail server records. Time stamps are often given in the following format:
hr:min:sec +/- GMT
where hr is hour, min are minutes, and sec are seconds. +/- GMT is the Greenwich Mean Time difference which can be positive (+), negative (-), or neutral (0).
2. To compute the time difference between the two records, do as follows:
Step 1: Convert time stamps to GMT times.
Step 2: Convert the new times into seconds.
Step 3. Take differences.
You should be able to convert this difference back to a hr:min:sec format
This can give you an idea of the estimated email delivery time between the two records; that is, if you trust email headers information.