This is a very intersting question to consider, as really to answer it we need to move towards some kind of definition of what is meant by the expression "metaphysical." In a sense, this word is a label that has come to be applied to the work of certain poets, Donne of whom is one, who write poems that are short, draw their subject matter from the big issues facing their age, and are characterised by a rejection of traditional forms of expression and the adoption of startling conventions, particularly in the area of imagery. The conceit, in particular, which makes improbably comparisons, seems to be above all else the label that signifies whether a poet's work can be considered to be "metaphysical" or not.
To give one example of one of John Donne's conceits, let us consider "A Valediction: Forbidding Mourning," where a dying husband compares the link that exists between himself and his wife to a pair of compass points that are always linked even when they are separate:
Our two souls therefore, which are one,
Though I must go, endure not yet
A breach, but an expansion,
Like gold to airy thinness beat.
Such original and striking imagery which elaborates its point beautifully explores and builds upon the central theme of the poem and also gives ample proof of why Donne is considered to be a metaphysical poet.