How do I specify my affiliation/contact information when I changed institutions and fields? I am in a situation similar to that of this question where I did some work with a former employer, I changed fields, and now something related to that work is in the process of being published.  Initial submission of the work was during my time with my former employer, but revisions are happening after leaving.  The paper is a single author paper.  Based on the responses to that question, it seems like I should be using my former employer as my affiliation, but I have a few difference in my situation that I would like to clarify.


The work in question is a discussion paper which was solicited from me by a journal.  As such, I gained the expertise necessary to write the paper working for my former employer, but the paper itself was written in my personal time so neither organization directly funded any of the work in the paper.  However I would not have been requested to write the paper if not for my work with my former employer.
If I do decide to use my former employer as my affiliation, what should my contact email be?  I would rather not use a personal email, but I don't have access to the work email from my former employer, and it seems weird for my affiliation to be Company A, but my email to be me@CompanyB.com.


In case it affects your response, the paper is in the field of quantitative forecasting and specifically on the use of machine learning for forecasting, which is what I did for my former employer.  I now work using machine learning for video processing on medical devices.
If you have the opportunity to be clear (that is, some limited online form doesn't prevent it) explain the transition "Jon Barker, formerly of Acme Corp, now with Zypppy Inc. JB@Zypppy.com". Or something similar. 

The reason for the email is so that people can contact you, so use the current one even if it seems weird. Using an obsolete mail does no one any good. 

If the former employer/institution contributed funds to the project it is usually good etiquette to indicate that you were associated with them then. But for identification purposes, your current affiliation is more valuable. But if it is a paper with multiple authors, some still at Acme, then it is less important to list an old affiliation (my opinion only). 

There may even be reasons that you must not imply an affiliation that no longer exists, but that is fairly rare. If you can only give a single affiliation, it is probably best in most cases to list your current one. I suspect such a rule has many possible exceptions, but I'd suggest thinking about why you think you have an exceptional case.