mount_smbfs: server rejected the connection: Authentication error on Mac OS High Sierra 10.13.6
According to its man page, mount_smbfs takes its share point argument in the form:

//[domain;][user[:password]@]server[/share]


Note the "user[:password]" part -- the colon and password are in the same brackets, indicating that they're optional but if included, they must be included together. Essentially, if you include the colon, whatever's after it (up to the "@") will be taken the password. But you have nothing after the colon, so you're explicitly specifying a blank password.

Also, the man page says you should never run mount_smbfs directly, but instead use mount -t smbfs.

You need to either include the password explicitly, like this:

mount -t smbfs '//share;user:password@server.domain/share' /Volumes/share


Or leave off both colon and password:

mount -t smbfs '//share;user@server.domain/share' /Volumes/share


...in which case it'll look for a password in ~/Library/Preferences/nsmb.conf, and if it isn't there it'll prompt for one. I had thought that it could look in the keychain, but apparently it doesn't know how to do that. This means that there is no non-interactive, secure way to supply a password to mount_smbfs.

Depending on the context this is running in, you might be able to use the open command instead:

open 'smb://share;user@server.domain/share'


I think this'll need to be running in a user session, and doesn't give you control of the mount point (it'll be auto-created in /Volumes).

Oh, one other thing: the "share;" part of the URL specifies an authentication domain to find the user in. Is that part actually correct? If it is, the open command should work interactively.