If users tweet to enter a contest and mentions a twitter handle and hashtag, is it against the automation policy to reply all those people and let them know that their submission was accepted with a link to the contest gallery?
You’re probably on good ground if the users you are replying to @mentioned the account you use to reply with. For best results, utilize in_reply_to_status_id pointing to their entry submission. Keep in mind you still need to keep within the tweet allowances. Best to vary your messages a bit as well. Be careful not to just mention anyone using the hashtag, as use of a hashtag does not warrant an invitation to an at-reply.