You must be using it wrongly.
replace replaces a substring with another string, extending or reducing the string size as appropriate.
This is to do with the emails, isn't it?
The best way of seeing what it does is to run some test code to get used to how to manipulate the strings, and to check that the substitution works correctly.
Maybe you somehow are using a fixed length string.
Either
newstring = replace(oldstring, "searchfor", "replacewith") OR
oldstring = replace(oldstring, "searchfor", "replacewith")
should replace a string of 9 characters with a string of 10 characters, and the result should be one character longer.
Obviously you will use variables not actual strings in your code.
For instance, I use xxxXXXxxxXXX in some labels to allow for an email address of any length and then use this to adjust the label. Note the upper case/lower case is not significant, It was just an easy way for me to count how many characters I had.
mylabel,caption = replace(mylabel.caption,"xxxXXXxxxXXX",emailaddress)