This post on Filemaker Community may be of interest.
It was the first in the second link group in this Google https://www.google.com/search?q=webdirect+smtp+email
The third link in that group indicated a known problem in v13 was fixed in FMS v14
I had this problem and I thought that I had tried all ports, but the solution for us was to change to port 587 TLS encryption and plain password. Not been a problem since.