Para descobrir o que você pode fazer para evitar o envio de e-mails para a pasta de SPAM, consulte este Teste do servidor de e-mail
Como configurar o servidor SMTP no IIS de forma que os e-mails enviados do meu servidor não passem para a caixa de spam do gmail, yahoo ou hotmail?
Para descobrir o que você pode fazer para evitar o envio de e-mails para a pasta de SPAM, consulte este Teste do servidor de e-mail
Isso é um pouco difícil de entender. Eu acho que há duas maneiras de interpretar sua pergunta com duas respostas muito diferentes. Vou tentar dar os dois significados que vejo e uma resposta para cada pergunta abaixo.
1. Is there a setting I can choose on IIS's SMTP server (or any other mail server for that matter) that will stop other people marking email sent from my server as junk.
No... There are things you can do with DNS settings to manage email reputation (spf, etc) but ultimately this is a choice made by the receiving server's operator, and also by their users.
2. Help, I think my server is being exploited by spammers, because the legitimate mail I'm sending is being rejected with messages that indicate I have an open relay or am participating in sending spam. What do I do?
In general terms, you need to make sure that
- You are only accepting email to send through this server from your own internal processes... in other words that your email server is configured to only accept and forward email from servers you control.
- That these processes are secure - there was a spate of people exploiting issues in email forms on webpages, for example, to get around the problem I mention above.
- And any authentication you use to connect from the process you want to send email via this server hasn't been compromised.