We note that always EHLO command sent by sTunnel to SMTP server always been populated as "localhost". Note some well know SMTP server (or) SMTP relay servers such as smtp-relay.gmail.com, etc reject further communication with SMTP clients (as here sTunnel), if they receive initial handshake EHLO request itself as LOCALHOST.
Please refer this link on why SMTP servers/ relay servers reject EHLO command with LOCALHOST - https://support.google.com/a/answer/2956491. Note because of this reason, we could use sTunnel as our encryption channel to communicate with SMTP server/relay server.
Is there any specific reason behind in hardcoding the EHLO command with LOCALHOST always even in latest version of sTunnel - 5.57 version..? Always what would be in sTunnel functionality, if we change the EHLO command to populate as hostname (or) FQDN instead.