SYNTAX

At the first level of validation (SYNTAX), the syntax of email expressions is checked and even both non-strict conditions and international characters (except TLD) are allowed.

DOMAIN

At the second level of validation (DOMAIN) - which is the default one a DNS system is queried for domain validity and mail exchange server information. The following four attributes can be set to optimize the ratio of performance to false-negative responses: Domain cache size, Domain retry timeout, Domain retry count and Domain query A records. The number of queries sent to a DNS server is specified by the Domain retry count attribute. Its default value is 2. The time interval between individual queries that are sent is defined by Domain retry timeout in milliseconds. By default, it is set to 800 milliseconds. Thus, the whole time during which the queries are being resolved is equal to Domain retry count x Domain retry timeout. The results of queries can be cached. The number of cached results is defined by Domain cache size. By default, 3,000 results are cached. If you set this attribute to 0, you turn the caching off. You can also decide whether A records should be searched, if no MX record is found (Domain query A records). By default, it is set to true. Thus, A record is searched, if MX record is not found. However, you can switch this off by setting the attribute to false. This way you can speed the searching two times, although this breaks the SMTP standard.

SMTP

At the third level of validation (SMTP), attempts are made to connect SMTP server. You need to specify the number of attempts and time intervals between individual attempts. This is defined using the SMTP connect attempts attribute. This attribute is a sequence of integer numbers separated by commas. Each number is the time (in seconds) between two attempts to connect the server. Thus, the first number is the interval between the first and the second attempts, the second number is the interval between the second and the third attempts, etc. The default value is three attempts with time intervals between the first and the second attempts equal to 1,000 and between the second and the third attempts equal to 2,000 milliseconds.

Additionally, the EmailFilter component, at SMTP and MAIL levels, is capable of increasing accuracy and eliminating false-negatives caused by servers incorporating graylisting. Graylisting is one of very common anti-spam techniques based on denial of delivery for unknown hosts. A host becomes known and "graylisted" (i.e. not allowed) when it retries its delivery after specified period of time, usually ranging from 1 to 5 minutes. Most spammers do not retry the delivery after initial failure just for the sake of high performance. EmailFilter has an anti-graylisting feature which retries each failed SMTP/MAIL test for specified number of times and delays. Only after the last retry fails, the address is considered as invalid.

MAIL

At the fourth level (MAIL), if all have been successful, you can send a dummy message to the specified email address. The message has the following properties: Mail From, Mail Subject and Mail Body. By default, the message is sent from CloverDX <clover@cloverdx.com>, its subject is Hello, this is a test message. And its default body is as follows: Hello,\nThis is CloverDX test message.\n\nPlease ignore and don't respond. Thank you and have a nice day!