How to use vimium to select text from a page

If you specifically want to search for mailto but then select verycomplicatedandlong... i.e. search one string in order to find the second (unknown) string, which you then want to select.

/ mailto Enter / veryl Enter v ww y

Shortcut explanation.
/ - Enter search mode.
mailto - Literal search string.
Enter - Normal mode.
/ - Now that you know your second search string, enter search mode again.
veryl - Literal search string.
Enter - Enter normal mode (optionally you can use nN to find other instances of the same search string).
v - Enter visual mode.
ww - Select two words (domain and TLD) or use hl to select one character at a time.
y - yank (copy).

As a slight improvement on Filip's answer, I would suggest searching for mailto like he suggested, but then going into visual mode right away, then using w or e to select text to the end of the address and then using ow to switch to the marker at the beginning of the word and moving it forward. So the full command would be: / mailto Enter v ee ow