Postmark – Transactional Email Delivery. Customer.io - Email connected to your web or mobile app. Userfox, better welcome emails. Targeted email marketing software. Send personalized email campaigns to your customers with Vero. SMTP Relay Server: App and Transactional Email Delivery by SendGrid. Transactional Email from MailChimp - Mandrill. Solution de Cloud Emailing - Service SMTP & Newsletter - Mailjet. Email Delivery & Transactional Email Service.