It’s increasingly common for emails sent to Hotmail, Yahoo and AOL to be returned (‘Returned mail: see transcript for details’) as Internet Service Providers introduce increasingly simplistic measures to combat spam, e.g. simply blocking one RBL list or IP range.
Unless your outgoing email server is deemed 100% trust worthy you can’t guarantee the delivery of genuine emails.
Even if you set-up your outgoing email server with all the correct credentials (PTR, SPF DXIM etc.) there can still be problems as some ISPs will simply not accept emails from servers with IP numbers within a customer range of another ISP; at the time of writing if you are a BT Customer then ISP 1&1 will not accept any email!
To avoid delivery problems there are two recommended solutions:
- Use the SMTP server of your ISP.
- Use a third party SMTP Server provider e.g. http://www.arrowmail.co.uk/services.aspx#h1
Maintaining the trustworthiness of a sending server is an on-going job. It involves keeping up with the latest “good behaviour” standards, being fully RFC-compliant, staying off blacklists and requesting ISPs to trust your server. Unless you want to do this maintenance then use one of the suggestions above.