For people signing up, sure. Anything that can send emails containing user-generated content will get abuse that way. But for logging in, it seems odd; unless y

For people signing up, sure. Anything that can send emails containing user-generated content will get abuse that way. But for logging in, it seems odd; unless you require something like it for SMTP access (which I haven’t heard of anyone ever doing), it’s not going to help you block spam-senders.