I have few html email templates which are used to send mass emails to Contact and Campaigns. They are working fine in Salesforce classic whereas in Lightning, they are delivered incompletely. We need to send this template completely as there are few links on the template where recipients can subscribe. I didnt find any article online supporting this issue. Is there any workaround or can anyone suggest how to send template completely in Mass email in Lightning.
Based on your clarification the problem only exists with list emails on Campaign and Contact, I think I found the answer to your question as to why you're having the problem you're seeing. List emails in Lightning are limited to 32,000 characters.