The SMTP service has the ability to block inbound and outbound emails over a certain size. Users may get the error "552 Message size exceeds fixed maximum message size" when trying to send an email that is over the size limit.
It is possible to set a soft and hard limit on the size of an inbound message under the properties of the SMTP Connector. This setting is found under the SMTP Inbound settings, by clicking the Settings button under the Advanced Settings heading. You can set the advertised maximum message size, which will tell clients the maximum size supported. Some clients or automated software may ignore this value, so there is an "Enforce this message size" option to make sure the limit is not broken.
It is also possible to configure a hard
limit for the maximum outbound message size. This is configured under the
SMTP Outbound settings. If this option is configured, any attempt to send a
message that exceeds the limit will be rejected and the originator will be
notified. This option is useful if messages are put in the queues without going
through the SMTP inbound.