It seems you are processing personal data (email addresses are identifiable). Then you do have to comply with the GDPR. But what you are trying to do can be done compliantly – this doesn't have to be a GDPR violation.
Core question would be who the Data Controller is – who determines the purposes for which this data is processed. It seems that in this B2B scenario, you wouldn't be the Controller but merely a processor. This can simplify your compliance requirements, for example you wouldn't be responsible yourself for responding to data subject requests. You would however have to sign a data processing agreement with the Controllers (your clients), and can't hire subprocessors on your own. The relevant details are in Art 28.
Note that where the Controllers are not subject to the GDPR (for example because they are established outside of the EU and aren't processing data from persons in the EU), then merely processing the data in the EU doesn't make this processing subject to the GDPR. Whether GDPR applies depends solely on the relationship between the Controller and Data Subject, not on the location of the Processor.
Things are different if you determine the purposes of processing and would therefore be a Controller. This might be the case if you want to combine data from multiple sources, or if you are using the data for your own purposes. It would also be the case if you accept data directly from Data Subjects. But such a scenario would be more complicated for everyone involved. For example, your clients (who are Controllers as well) would now need a legal basis that allows them to share the data with you.