One or messages scanned by Messaging Gateway (SMG) receive the "Unscannable due to limits exceeded" verdict but the message and attachment sizes are all withing the configured message and attachment size limits.
Brightmail Engine / bmserver_log
2022-04-27T15:34:21+02:00 (ERROR:12882.1728042752):  [ <[email protected]>] SG Decomposer: Maximum allowed compression ratio exceeded.
2022-04-27T15:34:21+02:00 (ERROR:12882.1728042752):  [ <[email protected]>] SG Decomposer: Maximum allowed compression ratio exceeded. File(s) were not extracted from [empty filename].
One or more attachments have exceeded the upper limit on file compression. The file compression limit protects the SMG message scanning enging from denial of service attacks via "zip bombs" i.e. attachments which are heavily compressed and expand consume large amounts of system resources.
Release : 10.7.3
Component : Default-Sym
Currently the maxium compression ratio limit for the message decomposer cannot be configured or modified.