Hello RobertH1249,
Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
Based on the logs for your account, it looks like the scheduling requests are failing with a 500 response because the trial account has reached its sending limit of 100.
While trial accounts created via the Developer portal do not expire, they are still held to the same limited sending capabilities of a standard trial account. For example, you could send one email to 100 contacts, 100 emails to 1 contact, 20 emails to 5 contacts, etc.
Constant Contact free trial
https://knowledgebase.constantcontact.com/articles/KnowledgeBase/5549-constant-contact-free-trial
Please have a look and let us know if you have any other questions!
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