This assumes your data is in row 1 and that there is always a first
name and a last name:

=A1&"."&IF(ISBLANK(B1),"",B1&".")&C1&"."&"@"&D1

wrote:
Excel MVPs:

So this is another question about figuring out emails. I have first
name in column A, Middle Initial in Column B, and Last Name in Column
C, and Domain in Column D. I need a formula that gives A.B.C@D If there
is a middle initial, and A.C@D if there isnt one. Thanks again.

Reece