I found a solution but the cause is still unknown to myself. Amending mailhost parameter, within ns-config.tcl, allowed a different content, wraped in the email source. It seems ad_proc acs_mail_lite::smtp connects correctly to Postfix now. Emails from OpenACS work properly now.
Why assiging the domain would be a cause, if hostname was properly configured in the server (i.e. OS settings) ?
[30/Nov/2018:00:28:46][17022.7f44632ce700][-conn:iurix:1:5-] Notice: SEND EMAIL to_addr mailto:iuri_sampaio@hotmail.com
[30/Nov/2018:00:28:46][17022.7f44632ce700][-conn:iurix:1:5-] Notice: -multi_token ::mime::13
-headers {From mailto:nevessouza@iurix.com} {Reply-To mailto:nevessouza@iurix.com} {To mailto:iuri.sampaio@gmail.com}
-originator mailto:bounce-879-2663A53C9FB1055B413E8067C1C985EAB0B85CC7-282@iurix.com
[30/Nov/2018:00:28:46][17022.7f44632ce700][-conn:iurix:1:5-] Notice: ORIGINATOR mailto:bounce-879-2663A53C9FB1055B413E8067C1C985EAB0B85CC7-282@iurix.com
[30/Nov/2018:00:28:46][17022.7f44632ce700][-conn:iurix:1:5-] Notice: send cmd: smtp::sendmessage ::mime::13 -originator mailto:nevessouza@iurix.com -header {From mailto:nevessouza@iurix.com} -header {Reply-To mailto:nevessouza@iurix.com} -header {To mailto:iuri.sampaio@gmail.com} -servers localhost -ports 25
Nov 30 00:28:46 iurix postfix/smtpd[17012]: connect from localhost[127.0.0.1]
Nov 30 00:28:46 iurix postfix/smtpd[17012]: 34DF04C93E: client=localhost[127.0.0.1]
Nov 30 00:28:46 iurix postfix/cleanup[17016]: 34DF04C93E: message-id=< mailto:17022.1543537726.8@iurix.com >
Nov 30 00:28:46 iurix postfix/smtpd[17012]: disconnect from localhost[127.0.0.1] ehlo=1 mail=1 rcpt=1 data=1 quit=1 commands=5
Nov 30 00:28:46 iurix postfix/qmgr[16715]: 34DF04C93E: from=< mailto:nevessouza@iurix.com >, size=1176, nrcpt=1 (queue active)
Nov 30 00:28:46 iurix postfix/smtp[17017]: 34DF04C93E: to=< mailto:iuri.sampaio@gmail.com >, relay=smtpout.secureserver.net[173.201.192.229]:25, delay=0.44, delays=0.06/0/0.26/0.11, dsn=2.0.0, status=sent (250 SWfmg0lQwGkjo mail accepted for delivery)
