You need to evaluate how the email address is composed. In fact, a general email address like info@... is not considered as personal data, so you can send emails to companies to present your services, while email addresses with name or surname (direct contact) are personal data under GDPR.
How did you collect emails? Did website users left it to you on your website (maybe filling a form?) In this case check your email notification and be transparent with your visitors by informing them that you will use their email address to send them news about services provided and other great content.
If you collected email on social networks or companies websites, you need to be aware that these contacts are not published to receive advertising or promoting contacts, the person who published it does not attend to receive such email (unless the text around it allow you to believe otherwise) so that it would not be appropriate to contact them. However, if you model your email as a cold email using legitimate interest as a legal ground, you can present your company and the advantages in working with you and underline that you are contacting them because they are looking for a similar profile. This would be in line with GDPR requirements.