Like most banks, we have customers who do not have an email address on file. We want to send an email advertisement about a personal line of credit product (of course following CAN-SPAM laws) but are we also required to mail an advertisement to those customers without an email address on file? I want to avoid the cost of printing and postage for a mailed advertisement if at all possible.

Thanks in advance for any guidance.

You won't find specific requirements about marketing to anybody. As with any marketing campaign, you need to evaluate your overall marketing program for potential fair lending and disparate impact risk.
If you only market a particular loan product via email, what are the risks of excluding a protected class? For example, if the majority of your customers not receiving marketing for a particular product are over a certain age because that demographic does not utilize email could increase your fair lending risk of age-based discrimination.

Welcome to BOL! Good question.

Agree with Brian, and his references hit the point.

Examiners look at results, which are determined in many cases by outreach.
If a higher proportion of 62+ individuals don't have internet (because they were not brought up with it)
or a higher proportion of minorities are below the average income levels (and can't afford internet),
or your marketing does not address a minority group because the ad is not in their language (and they don't understand it)
or images do not match the demographics of your community (not making the recipient feel welcomed)

it could all be reasons for an examiner to look at the "Effects Test" i.e., you are not discriminatory in your marketing, but the actions have the effect of being discriminatory.