Summarize Email field in Table Relationship

np
The combined text summary field will summarize any text field. When you have a situation where you need to roll up words from a field that is not actually a text field just make a Mirror field like [email (text format)]

with the trivial formula of just

[email]

you can just roll up that text field in the upper direction and of course in the down direction you can just use a look up. Note also that if you need to get a combined text summary field rolled up from a grandchild to a parent then up to a grandparent, the other field type that can be used in a combined text summary field is actually a combined text summary field.