And every Blossom on the Bush Adjusts its tumbled Head –
While the first four lines focused on a vague occurrence, here the poem shifts to something more concrete (as in, more detailed, not more cement-like).
Suddenly, thankfully, the speaker mentions flowers on a bush.
What about these flowers? Well, they seem to be reacting to this colorful swirl of movement. Of course, flowers don't really have "heads," no matter how "tumbled," or messy they might look. Nor are they capable of adjusting those heads. Here, the speaker is using personification to give these inanimate flowers human qualities.
Why might she do that, though? Well, giving the flowers some human quality underlines how they are reacting (as people might) to the presence of this very colorful source of motion. It's as if this presence has some sort of power over the flowers.
The mail from Tunis, probably, An easy Morning's Ride –
Whatever bumped into the flowers is just incredible to the speaker. She imagines perhaps that it comes from some faraway place like Tunisia (also called "Tunis"), which is in Africa.
The speaker wouldn't put such a long trip past the subject of her poem. What's more, the subject could make such a long trip in no time flat. Fly to Tunisia you say? Easy peasy. I'll see you in time for lunch. The speaker is clearly impressed by what she's describing.