Just in case other people run into trouble with this:
I defined a newsletter subscription form in FSCF which would send a mail to a
firstname.lastname@example.org email address, which a typical way to subscribe to an EZMLM mailinglist which a lot of webhosts offer on their servers.
I tested with my own email address in BCC and the mail was send without a problem, but I never received the expected confirmation email from EZMLM and the subscriber email address was never added to the mailing list.
Tried all the different mail sending methods and numerous other things. I had this working on a non-WP site elsewhere using PHPmailer, so I compared the mail headers which were being send and found that the
Return-Path: header was the problem. For some reason that reverted to a server email address when using FSCF instead of being set to the subscribers email address.
On the non-WP site I used PHPmailer with SMTP enabled. So I installed the WP SMTP plugin and tried again and lo and behold, it worked!
The relevant settings are:
In FSCF use mail function 'WordPress'.
In the SMTP settings I just used:
* Send all WordPress emails via SMTP
[v] Set the return-path to match the From Email
No encryption, no authentication and all other fields left blank
I hope info this helps others who may run into this issue.