What is the requirement of sending emails that will not be blocked by spam filters

Email(toEmailAddress, fromEmailAddress, subject, body, fileName);

I use SmtpClient class to send emails. The hard coded sender's address is donotreply@somedomain.com.

I have run some tests with different "TO" email addresses. Emails sent to my hotmail, and Gmail are never sent to my company开发者_如何学Python's email address even to junks. Neither is our client's email address.

What is the requirement of sending emails that will not be blocked by spam filters?

Hotmail (and others) block emails sent from IP-addresses marked as "private". That is addresses that are assigned to ISPs' who in turn, through DHCP, assignes them to customers. Since these addresses can be assigned to different customers at different times they are not static per computer/server. This is something spammers have utilized in the past (and still do) to avoid spam-detection.

Hotmail (and others) try to limit the amount of spam by denying relaying from private addresses.

So, there are a number of factors to consider here:

1. Where you are sending it from can factor. There are source IP ranges that are outright blocked by email servers (or at least, email servers that are secure and not spam forwarders) Check out http://www.spamhaus.org and validate your source IP to confirm you are not automatically rejected on send.
2. Check out https://col113.mail.live.com/mail/troubleshooting.aspx and https://col113.mail.live.com/mail/services.aspx - these are good starter points to understand how ISPs like Hotmail recommend mail to be sent to them.
3. Make sure you do well-formed sender, recipient, subject, and body fields. Poorly constructed messages can often be a sign of spam. Also use clean display names with the emails John Smith . One of my build systems was going straight into Outlooks junk filter until I cleaned up the display names and ensured that my subject and body were cleanly written (with good casing/etc).

The important thing to remember is that spam filters are looking for not just poorly formed messages, but automated messages and highly repetitive messages sent to many senders. I would be cautious about "donotreply@" unless you have carefully read (1) and (2), because at this point in time there may very well be some heuristics written around obviously fake addresses like those as well. (or else every spammer would just do donotreply@extendyour.org)

There are no conditions that guarantee a message will pass through a spam filter. Think about it: if such conditions were to exist then spammers would make sure their messages satisfied the conditions -- which would make the spam filters pretty much useless.