Combine Array with String

The ForEach-Object cmdlet requires pipeline input, which you're not providing (in its absence, the script block is called once, which means there is no point in using ForEach-Object at all).

Also, since you need per-element processing of your $split array, you must enumerate it, which the pipeline does for you automatically, as Jeroen Mostert's comment implies:

# Enumerate the elements of array $split, i.e. send
# its elements one by one to ForEach-Object, which sees each as $_
# Note: You can capture the results in a variable simply by 
#       uncommenting the next line:
# $emailAddresses = 
$split | ForEach-Object { $mail + $_ + $domain }

Note the use of the automatic $_ variable, which refers to the current pipeline input object in each iteration.

However, with values already in memory it is more efficient to use the foreach statement:

# Note: You can capture the results in a variable simply by 
#       uncommenting the next line:
# $emailAddresses = 
foreach ($index in $split) { $mail + $index + $domain }

Note the need to specify the input as part of the (...) expression, and the need to name a self-chosen enumeration (iterator) variable.

Finally, a concise and efficient but somewhat obscure solution is to use the regex-based -replace operator:

# Note: You can capture the results in a variable simply by 
#       uncommenting the next line:
# $emailAddresses = 
$split -replace '^', $mail -replace '$', $domain

This relies on -replace being able to operate on arrays as input, in which case the replacement operation is performed on each element, resulting in a transformed array as output.

Regex ^ matches the start of a string, $ matches the end, so that the two operations effectively prepend / append the value of $mail / $domain to each element of $split.

And you can user variable for pipelines in mklements0's solution:

$elements = $split | ForEach-Object { $mail + $_ + $domain }
# [email protected]
Write-Host $elements[3]