Sometimes the SEO is a subcontracted service in another country, so engaging directly with the ultimate customer is not an option.
Depending on the size of the customer's business, it might not be feasible to get an email account for the SEO agency. e.g. I work at a large company and can only use company-supplied devices to access email and it can't be forwarded (without a lot of red tape).
Skype offers phone numbers, which could be one option.