- Avoid any string that looks like spam.
Most Spam checking these days is Bayesian, which means that that your
message is checked using a fuzzy algorithm that tries to guess if
resembles known Spam or Ham (good) messages (mainly by checking the
frequency of common spam words and phrases).
- Send individual messages to each recipient instead of copies.
It is better to send an individual message to each recipient, rather
than using multiple addresses in the BCC field because many spam
filters (and many ISP's) automatically flag multiple recipients as
- If possible send via your ISP's mail server rather than using a local SMTP Server.
Messages sent from a mail server running on your computer may be
flagged as spam because some mail servers will try to contact the
source IP of the sending server (which will fail with a local IP
- Try with smaller batches of e-mails.
It would appear that some of the big mail hosts such as Hotmail will
recognize when an identical message is sent to a large number of
subscribers at one time so you should stagger the delivery of your
messages [...] to send your messages in small batches.
- Minimize your use of attachments.
- Make sure the computer sending the email has a Reverse PTR record.
What's a reverse PTR record? It's something your ISP has to configure
for you -- a way of verifying that the email you send from a
particular IP address actually belongs to the domain it is purportedly
- Configure DomainKeys Identified Mail in your DNS and code.
What's DomainKeys Identified Mail? With DKIM, you "sign" every email
you send with your private key, a key only you could possibly know.
And this can be verified by attempting to decrypt the email using the
public key stored in your public DNS records.
- Set up a SenderID record in your DNS.
To be honest, SenderID is a bit of a "nice to have" compared to the
above two. But if you've gone this far, you might as well go the
distance. SenderID, while a little antiquated and kind of..
Microsoft/Hotmail centric.. doesn't take much additional effort.
SenderID isn't complicated. It's another TXT DNS record at the root
of, say, example.com, which contains a specially formatted string
documenting all the allowed IP addresses that mail can be expected to
Sources and additional information: