According to Unitarians, to whom does the word הָאָדוֹן refer in Mal. 3:1? Regarding the word אֲדֹנִי (adoni) in Psa. 110:1 (actually prefixed with -ל, i.e. לַאדֹנִי), translated as “my lord,” a self-professed Unitarian website states,1


  Trinitarian commentators frequently argue that “my Lord” in this verse is the Hebrew word adonai, another name for God, and is therefore proof of the divinity of the Messiah. But not only is this not a valid argument, this verse is actually one of the great proofs of the complete humanity of the promised Messiah. The Hebrew word translated “my lord” is adoni (pronounced “Adon nee” 1 ) in the standard Hebrew texts. This word is always used in Scripture to describe human masters and lords, but never God. (emphasis mine)


Now, the Hebrew word אֲדֹנִי is simply the lemma or base form אָדוֹן2 with a 1st person singular pronominal suffix appended (i.e., “my”). The question, then, should not be limited to whether אֲדֹנִי is ever used in reference to Yahveh (God), but rather, whether אָדוֹן and any of its suffixed forms (including אֲדֹנִי) are ever used in reference to Yahveh. After all, couldn’t “lord,” “my lord,” “his lord,” “our lord,” and “their lord” all refer to the same “lord”? 


The lord said to Abraham and Sarah,...
Abraham said to his lord,...
Sarah said to her lord,...
Sarah and Abraham spoke to their lord,...
Sarah and Abraham said, “Our lord...”
Abraham said, “My lord...”


In this context, the “lord” is the same person, regardless of the pronominal suffix. Hence, limiting the discussion to “my lord” is both arbitrary and disingenuous.

I encountered the word הָאָדוֹן in Mal. 3:1. This is simply אָדוֹן prefixed with the definite article הָ, meaning “the lord” (i.e., “the master”).

The Hebrew text of Mal. 3:1 states,


  הִנְנִ֤י שֹׁלֵחַ֙ מַלְאָכִ֔י וּפִנָּה־דֶ֖רֶךְ לְפָנָ֑י וּפִתְאֹם֩ יָבֹ֨וא אֶל־הֵיכָלֹ֜ו הָאָדֹ֣ון אֲשֶׁר־אַתֶּ֣ם מְבַקְשִׁ֗ים וּמַלְאַ֨ךְ הַבְּרִ֜ית אֲשֶׁר־אַתֶּ֤ם חֲפֵצִים֙ הִנֵּה־בָ֔א אָמַ֖ר יְהוָ֥ה צְבָאֹֽות׃


which is translated into English as,


  Behold, I am sending My messenger, and he shall prepare a way before Me, and the lord whom you are seeking shall suddenly come to His temple, and/even the messenger of the covenant whom you delight in. Behold, he comes," said Yahveh of hosts.


We note a couple of things during exegesis of Mal. 3:1:


Yahveh of hosts is the speaker.


“I am sending My messenger” evidently refers to Yahveh sending His messenger (cp. 2 Chr. 36:15-16; Isa. 42:19).
Thus, first-person pronouns refer to Yahveh, such as “My messenger,” “before Me,” etc.



However, note the acute analysis of Carl Friedrich Keil in his commentary on Mal. 3:1:


  “The Lord” ((hâ'âdōn)) is God; this is evident both from the fact that He comes to His temple, i.e., the temple of Jehovah, and also from the relative clause “whom ye seek,” which points back to the question, “Where is the God of judgment?” (Malachi 2:17).


The question: How do Unitarians refute the assertion that הָאָדוֹן (ha-adon) refers to Yahveh (God) in Mal. 3:1?



Footnotes

1 URL: http://www.biblicalunitarian.com/verses/psalm-110-1

2 meaning “master,” “lord,” “sir”

References

Keil, Carl Friedrich. Commentary on the Old Testament. 1900. Reprint. Trans. Martin, James. Vol. 10. Grand Rapids: Eerdmans, 1988. (458)
You ask &quot;How do Unitarians refute the assertion that הָאָדוֹן (ha-adon) refers to Yahveh (God) in Mal. 3:1?&quot;
The question seems to hold the hidden assumption that Unitarians need to refute the Trinitarian position in order to be a valid belief.
It also seems to hold the hidden assumption that Unitarians generally don't think that הָאָדוֹן (ha-adon) refers to God in Mal. 3:1.
I think both these assumptions are wrong. And so the short answer could be: &quot;They don't generally refute that assertion&quot;. As @Cannabijoy a Unitarian who provided the previous answer made clear.
A slightly longer answer would be that they don't need to be able to refute the Trinitarian position in order for their position to be a valid alternative position. They can just prefer it over the Trinitarian position which they might find un-intuitive and lead to unique interpretations where a person using a word that normally indicates a relationship to another is actually referring to their self. They might question how they could justify to God why they thought God would have expected them to make such novel interpretations.
Regarding the assertion that הָאָדוֹן (ha-adon) refers to God in Mal. 3:1 :
They accept that it does, but reject that הָאָדוֹן (ha-adon) is the messenger of the covenant. On the basis that a messenger is one that carries a message for another. They can view Jesus as the messenger of the covenant for example.
It seems to me that they could also reject that it does, but I don't understand the Hebrew grammar issues so could be mistaken here. But it seems to me they could think that those questioning where the God of Judgement in Malachi 2:17 meant it more in the sense of asking why hasn't God made himself apparent. And therefore weren't thinking it was a matter of seeking him. The Lord that the people seek הָאָדוֹן (ha-adon) in 3:1 was the Messiah, and the reason it states his temple is because the verse relates to 2:17 and the God of Judgement that hadn't seemed to make himself apparent. So that it could be read as something like &quot;And the Lord, whom ye seek, Will suddenly come to the God of Judgement's temple&quot;.
Those not being the only interpretations of that verse that would be available to them.