Message not sending with such settings
$mail = new PHPMailer;
$mail->SMTPDebug = 4;
$mail->isSMTP();
$mail->Host = "domain.com";
$mail->SMTPAuth = true;
$mail->Username = "login";
$mail->Password = "pasw";
$mail->SMTPSecure = "tls";
$mail->Port = 2525;
$mail->CharSet = 'UTF-8';
openssl extension is loaded. Dеbug output is here
https://justpaste.it/1ecjw
First problem line there is
2017-12-08 09:48:53 SMTP -> get_lines(): $str is "502 5.5.1 command not supported in "STARTTLS""
telnet says that there is ESMTP service. I don't know what to investigate next...
Not too complicated - your mail server does not support STARTTLS. If it did it would appear in the list of capabilities after the first EHLO command, which is this:
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250-8BITMIME
250-AUTH CRAM-MD5 PLAIN LOGIN
250-AUTH=CRAM-MD5 PLAIN LOGIN
250-XACK
250-SIZE 0
250-VERP
250 DSN
STARTTLS is not in that list. It may still support encryption via SMTPSecure = 'ssl' and Port = 465, but otherwise you'll need to fall back to the most secure auth option over this unencrypted channel, which is AuthType = 'CRAM-MD5'.
ESMTP just means that it supports "Extended SMTP" with EHLO (and all that that implies), not just basic SMTP HELO.
BTW - SMTPDebug = 4 is too noisy for this level of problem, you need 3 at most. Also, you're using an old version of PHPMailer, upgrade.