Recently we experienced a sudden increase in spam reports, and all of them come from Yahoo email addresses. We see lots of registration confirmation email got marked as spam. We also saw people marking mails as spam and then opened it and clicked on the confirmation link.
We send around 150 registration emails a day, and currently sees 2 spam reports from these per day. Previously spam reports once come once a month.
We use Sendgrid to send emails.
What are the possible reason for this, and what can be done to remedy it?