'Holism' has in recent years acquired a different meaning from the one Smuts intended. But the philosophy presented in Holism and Evolution in 1925 was genuinely novel under that name except under a reservation regarding John Tyndall (1820-93) :
to establish his thesis that the whole of creation is conditioned by
the existence of a universal tendency towards completeness, governing every
stage and every aspect of an evolutionary process in continuous operation.
The known facts regarding atomic structure, the association of atoms in a
molecule and of molecules, on the one hand in the unit structures from which
crystals are formed, or on the other hand in the colloidal state, enabled him to
discern a continuity of development throughout lifeless, but as he emphasizes
by no means inert, matter towards structures endowed with life. Having thus
bridged the gap between matter and life he had no difficulty in picturing the
development of mind in its most primitive form in a differentiated multicellular
organism. The next gap between mind and soul could be similarly bridged.
Life, mind and soul being thus conceived-as indeed Tyndall had contended
much earlier-not as something added to matter from without but as some-
thing arising from within matter the whole grand design of creation stands
revealed as something all-embracing, springing into existence in response to a
universal urge towards completeness. (Waverley, 'Jan Christiaan Smuts. 1870-1950', Obituary Notices of Fellows of the Royal Society, Vol. 8, No. 21 (Nov., 1952), pp.
271-273 : 272.)
I haven't an exact reference for Tyndall but Roy MacLeod, "John Tyndall," Dictionary of Scientific Biography, 16 vols. (New York: Scribners, 1970-1981), Vol. XIII, pp. 521-524 should be of use.
I cannot find evidence for correspondence between Smuts and Einstein 'during the period the theory of relativity was evolving' if this refers to 1905 (or earlier) and 1915. It is unlikely that Smuts could have made any technical contribution and hard to to make out what else any correspondence could have been about.
There is another angle, however.
Standing at the forefront of all major campaigns of the peace movement of the 1920s and early 1930s - among them disarmament, conscientious objection, and the formation of a European Union - Einstein positioned himself at the vanguard of an uncompromising anti-war activism. (Ofer Ashkenazi, 'Reframing the Interwar Peace Movement: The Curious Case of Albert Einstein', Journal of Contemporary History, Vol. 46, No. 4 (OCTOBER 2011), pp. 741-766 : 744-5.) Smuts was much involved in the interwar years (1918-39) in disarmament and international co-operation. So there was that degree of interlock between Einstein and Smuts.
There is a puzzling point which I can't resolve here. Ashkenazi writes : 'Einstein's 'immense authority' and his status of a 'great leader' are recurrently mentioned by his correspondents throughout the interwar years: for instance, General L.S. Smuts to Albert Einstein, 15 May 1936' (Askenazi, 744 fn 14). I know of no General L.S. Smuts and am pretty sure this is a misprint referring to General J.C. Smuts. But I don't have access to Einstein's correspondence for 1936, so this is conjectural.