As part of measure to prevent spam our mail server carries out a number of different checks on incoming mail. One or more of these may be the reason for email from certain addresses being blocked.
HELO FQDN & Sender verification Checks
When a mail server connects to our server the server carries out various "identity checks" using the information provided to ascertain whether or not this is correct.
The HELO Host check uses the provided host name of the connecting mail server and does a DNS check on that name to see if it matches the provided IP address. The server will also do a reverse DNS check (PTR) to see that IP address matches the Host Name.
If these identity checks fail, it is assumed that the email is spam / fraudulent and the message is rejected.
This can be fixed by the sender making sure their mail server HELO Host name is a valid FQDN and has a suitable A record in the domains DNS entries. The IP address should have a valid PTR record pointing to a FQDN with an A record with that IP Address.
SPF Check
The SPF check uses the details in the SPF DNS record, for a given email domain, to check that the sending email server is authorised to send email on behalf of that domain name.
RBL Check
All incoming email is checked to see if the mail server sending the email is on a Real Time Block list. If the domain/IP is listed, then the email is blocked.
TLD Check
Certain Top Level Domains such .guru, .cyou, .xyz etc are predominantly used for sending spam we block all email where the sender is using one of these TLD’s.
SPAM Check
We run a spam filter which checks the sender and contents of an email and gives it a spam score. If this score is considered spam the subject is marked as ***SPAM*** and if may be moved to a junk/spam folder.
