What is a Recipient Callout?

A recipient callout is a method of checking the validity of email recipients that is used in spam filtering and email security solutions. In a recipient callout, the recipient's email server is used to check whether the specified email recipient (recipient) actually exists on the server and can accept emails.

If a message is addressed to a recipient who is not known to the target server, there is no reason to accept this message. For this reason, the first step EuropeanMX takes when receiving a message is to check whether the recipient address exists on the target server. This is done automatically using a recipient callout. Existing and non-existing recipient addresses are stored internally in order to minimize the number of requests to the target server. We carry out the callouts at SMTP level, as all mail server types are compatible with this. An additional check of the user via LDAP is therefore not necessary.

How the recipient callout works at EuropeanMX

Our filter servers perform a so-called "null sender" callout as soon as a message is to be delivered. In such a callout, the filtering server connects to the target server and performs a query with an empty sender field ("< >", as in a bounce message) and the corresponding recipient address. After the recipient has been checked, the connection is closed. Please make sure that the target server supports recipient verification in this way.

To check whether the "Catch-All" function has been activated on the target server, the server is checked in advance with a randomly generated address. Of course, it makes no sense to carry out these callouts if the mail server accepts any mail, even if the recipient does not actually exist. The check result is saved on the filter servers as with a normal check.

Existing recipients

EuropeanMX automatically caches existing recipients on the respective filter host for up to 96 hours.

5xx Target server rejects

If the destination server permanently rejects a message for a specific recipient with a 5xx error, the destination address is marked as invalid and all messages for this recipient are rejected. This information is stored server-wide for 2 hours. The stored callout information can be removed at any time in the web interface via the menu item "Continuity" > "Empty callout cache - incoming".

Deactivate Recipient Callout completely

In rare cases, it may be necessary to deactivate the Recipient Callout of EuropeanMX. To do this, you can activate the option "Only accept email to your defined mailboxes" under "General" > "Mailbox configuration" in the "Configuration" tab. By activating this option, EuropeanMX will no longer perform a recipient callout, but will only accept messages for email addresses that have been set up in the "Mailboxes" tab. Before activating this option, you must therefore ensure that all valid e-mail addresses for your domain have been created under "Mailboxes". If an address is not entered there, EuropeanMX will reject the incoming message for this address.