If a good number of participants mark your messages as “spam” (or “junk” or “bulk"), which is referred to as an Abuse Complaint, then some of your future messages may go into the Junk/Spam/Bulk folder in that ISP.
If a high number of your participants mark your message as “spam”, this may result in a block with that ISP. Some blocks are temporary and last about 24 hours. Other types of blocks are permanent and require an escalation to that ISP.
Bulking and blocking essentially occurs when your mailing behavior results in a downgrade of your Sender Reputation. Having your messages not reach the inbox is one of the main factors of low response rates for your studies.
What happens when a reader hits "mark as spam"?
