Several old and new email clients have a bugs. To block messages with bugs, put in your acl_check_mime:
# Bounday Space Gap
drop condition = ${if match{$mime_boundary}{^( |\t)}{yes}{no}}
message = This message contains an broken MIME container (Boundary Space Gap). Boundary: $mime_boundary
delay = 45s
# Blank MIME Folding Vulnerability
drop condition = ${if match{$message_headers}{\N^\b$\N}{yes}{no}}
message = This message contains a broken headers (Blank Folding Vulnerability)
delay = 45s
# CLSID hidden extension
drop condition = ${if def:mime_filename {yes}{no}}
condition = ${if match{$mime_filename}{\N\{[a-hA-H0-9-]{25,}\}\N}{yes}{no}}
message = This message contains an unwanted CLSID hidden extension. Filename: $mime_filename
delay = 45s
# Empty MIME Boundary Vulnerability
drop condition = $mime_is_multipart
condition = ${if eqi{$mime_boundary}{}{yes}{no}}
message = This message contains a broken MIME container (Empty MIME Boundary)
delay = 45s
# Too Many MIME Parts
drop condition = ${if >{$mime_part_count}{256}{yes}{no}}
message = This message contains too many MIME parts: $mime_part_count (max 256)
delay = 45s
# Long MIME Boundary Vulnerability
drop condition = ${if >{${strlen:$mime_boundary}}{70}{yes}{no}}
message = This message contains a broken MIME container (Long MIME Boundary). Length: ${strlen:$mime_boundary}
delay = 45s
# Line length too long
drop regex = ^.{8191}
message = Line length in message or single header exceeds 8192.
delay = 45s
# Filename length too long (> 512 characters)
drop condition = ${if def:mime_filename {yes}{no}}
condition = ${if >{${strlen:$mime_filename}}{512}{yes}{no}}
message = Proposed filename too long: ${strlen:$mime_filename} characters (max 512 )
delay = 45s
# Boundary length too long (> 1024)
drop condition = ${if >{${strlen:$mime_boundary}}{1024}{yes}{no}}
message = Boundary length too long: ${strlen:$mime_boundary} characters (max 1024)
delay = 45s