Gmail webapp unfortunately. I did sales in a previous lifetime and cannot for the life of me quit all the gmail plugins I used. Specifically:

• Streak: CRM, schedule send later, snooze, read and link click receipt
• YAMM: mail merge
• Rapportive: recipient's linkedin on the sidebar

Nylas looks like it's going to meet my needs soon, but the app is just slows my old MacBook air down too much.