Problem with poller for mail inbound channel adapter

Hi,
i'm new in Spring integration.
I've configured an inbound-channel-adapter to receive mail and i set up a poller with fixed-delay = 15000.
When max-messages-per-poll is set to low value (less than the number of messages in the mailbox folder), the trigger fires correctly every 15 seconds.
But if max-messages-per-poll is set to a high value, Pop3MailReceiver is called every about 2 seconds and fixed-delay or cron settings are not considered.

Where is my fault?
Thanks in advance for your help.

Code:

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<util:properties id="javaMailProperties">
        <prop key="mail.pop3.socketFactory.class">javax.net.ssl.SSLSocketFactory</prop>
        <prop key="mail.pop3.socketFactory.fallback">false</prop>
        <prop key="mail.store.protocol">pop3s</prop>
    </util:properties>
       
        <mail:inbound-channel-adapter id="mailAdapter"
                  store-uri="pop3s://xxxxxxxxxx%40xxxxxxxxx.xxxxx:xxxxxxxxx@xxxxxxx:xxx/inbox"
                  channel="receiveEmailChannel"
                  should-delete-messages="false"
                  java-mail-properties="javaMailProperties"
                  auto-startup="false">
                    <int:poller max-messages-per-poll="10" fixed-delay="15000"/>
    </mail:inbound-channel-adapter>

        <int:control-bus input-channel="receiveEmailChannel"/>
   
    <int:channel id="receiveEmailChannel">
        <int:interceptors>
            <int:wire-tap channel="logger"/>
        </int:interceptors>
    </int:channel>
       
    <int:logging-channel-adapter id="logger" level="DEBUG"/>

    <int:service-activator input-channel="receiveEmailChannel" ref="mailNotificationDetector" method="receive"/>

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