JMETER a ${__RandomString function that is used for creating email addresses

mail format:

RandomString(COUNT_OF_CHARS,LIST_OF_CHARS,) + '@' + RandomString(COUNT_OF_CHARS,LIST_OF_CHARS,) + '.' + RandomString(COUNT_OF_CHARS,LIST_OF_CHARS,)

We can specify how many characters it should contain using function ${__Random(MIN,MAX,)}

So finaly it may look like:

${__RandomString(${__Random(3,9,)},abcdefghijklmnopqrstuvwxyz,)}@${__RandomString(${__Random(2,3,)},abcdefghijklmnopqrstuvwxyz,)}.${__RandomString(${__Random(2,3,)},abcdefghijklmnopqrstuvwxyz,)}

As per Using JMeter Functions guide __RandomString() function takes 3 parameters:

1. Length of the desired random string
2. Source characters
3. If you need to store generated string into a JMeter variable you can provide variable name as 3rd argument.

So to get line of 10 alphabet characters you can use __RandomString function as follows:

${__RandomString(10,abcdefghijklmnopqrstuvwxyz,)}