SMB Signing has been enabled by default by Apple dating back since the release of OS X 10.11.5 (15F34). 

SMB Signing digitally signs at the packet level of the SMB communication. This enables the receiver of the packets to confirm the point of origination and it’s authenticity. This security mechanism helps avoid issues like tampering and “man in the middle” attacks. As long as you're on a secure network, this should not be an issue. In addition, this causes slow performance and will not allow you to connect via SMB to the NAS Gateway. We also recommend you to disable SMB signing on clients that access a Vess R2600 to gain better performance on your macOS 10.11.5 - 10.13.4 clients.

Issues:

  1. Unable to mount volume using the SMB protocol with the NAS Gateway G1100.
  2. Authentication rejected with Internal and External LDAP users and only cifs:// allows mount and authentication.
  3. Slow performance via SMB with the Vess R2000 series.

To disable SMB signing which is enabled by default on macOS versions 10.11.5 -10.13.4, execute the instructions below by creating a nsmb.conf file:

sudo -s
echo "[default]" >> /etc/nsmb.conf
echo signing_required=no >> /etc/nsmb.conf
exit
 To check that it properly wrote the nsmb.conf file:
cat /etc/nsmb.conf
[default]
signing_required=no
 Check if SMB signing is disabled on your share after you remount the SMB volume:
#Display stats for a specific SMB share
smbutil statshares -m /Volumes/SMB_Volume_Name

#Display stats on all mounted SMB shares
smbutil statshares -a  
=====================================================================================
SHARE                         ATTRIBUTE TYPE                VALUE
=====================================================================================
SD01 
                              SERVER_NAME                   10.0.0.140
                              USER_ID                       501
                              SMB_NEGOTIATE                 AUTO_NEGOTIATE
                              SMB_VERSION                   SMB_3.0
                              SMB_SHARE_TYPE                DISK
                              SIGNING_SUPPORTED             TRUE
                              EXTENDED_SECURITY_SUPPORTED   TRUE
                              LARGE_FILE_SUPPORTED          TRUE
                              CLIENT_REQUIRES_SIGNING       TRUE
                              FILE_IDS_SUPPORTED            TRUE
                              DFS_SUPPORTED                 TRUE
                              MULTI_CREDIT_SUPPORTED        TRUE
                              ENCRYPTION_SUPPORTED          TRUE 
If the SIGNING_ON variable is still outputted via smbutil statshares -a, 
that means it's still enabled.
This variable should not show up after you have disabled SMB signing.
             SIGNING_ON                    TRUE
Related Article:

Article from Apple (HT205926)
Turn off packet signing for SMB 2 and SMB 3 connections

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