You are prohibited from reading this message unless and until you have read and agreed to the ****** Solicitors communication terms, which are deemed incorporated in this message.
Yes, that is right, you have to read (part of) the message to get to the bit telling you that you are prohibited from reading the message.
In order not to be prohibited you have to read and agree the terms which are incorporated in the message which you are prohibited from reading. So even if you consider that boiler plate text not to be part of the message, the terms are explicated stated as being incorporated in the message, the one you are prohibited from reading.
So to agree to the terms and not be prohibited from reading the message you absolutely have to have broken those terms already and ignored a prohibition.
AFAIK there is no way to "prohibit someone from reading a message". Even copyright law does not cover that, i.e. they have no "right" to prohibit reading a message and there is no consequence for ignoring their made up prohibition.
I cannot see how you could be deemed to have accepted their terms either, given that you have to ignore them in order to read and accept them in the first place.
Why do they do it?