Warning: Cannot modify header information

I’m getting the same problem, and I found out why by tracing the code in st_swift.php.

The reason you are getting this error message from st_swift.php is because [MY USERNAME] is not a valid email address. It has to be in an email address form(eg. [email protected]).

To work around:
1) open st_swift.php

2) go to line 117, the code of this line should be:
if ($swift->send($message, $to, $from)) {return true;}

3) On the above line of code, the variable $from is actually storing the [MY USERNAME] in our case; but again, it has to be in email format to make this line of code run correctly.
Therefore, we can:
a) hardcode an email address into $from by inserting
$from = “xxxx.anydomain.com”;
right before the 117 line of code(this is the solution i chose).
b) right before the 117 line of code, write a little code to check if $from contains the character “@”, if not, append “@anydomain.com” to it.

hope this helps.