The word of a snail on the plate of a leaf?
It is not mine. Do not accept it.

Acetic acid in a sealed tin?
Do not accept it. It is not genuine.
A ring of gold with the sun in it?
Lies. Lies and a grief.
Frost on a leaf, the immaculate
Plath’s speaker uses two  metaphors  to convey distrust: a snail trying to sound convincing on a plate of its favored food, and some acetic acid in an impossible place (acetic acid is usually formed by exposing alcohol to air). She does not trust the words of other people or possibly someone in particular. 
 Another reading suggests that “The word of a snail” feels small and insubstantial, meaning empty words that lack sincerity or promise. 
 Acetic acid is sour, pungent and corrosive, which could be harsh, baseless words only meant to hurt and demean. The ‘sealed tin’ could imply that he is cut off from her and what he has inside him is ‘acidic’ and hurtful. 
 Note the repetition of ‘do not accept it’. Plath has no doubt where she stands. In one line she says “It is not mine. Do not accept it.” She reverses this two lines later  when she says, “Do not accept it. It is not genuine.”  The similarity is known as   parallel syntax . The aim is emphasis and cohesion. 

The Couriers