I have exactly the same problem (osTicket v1.14.1 / nginx/1.16.1 / MySQL 5.7.28 / PHP 7.3.12) with Slovenian characters (ISO-8859-2). It works only if email encoding is UTF8. Here is the source mail.
Delivered-To: bla.bla@siol.si
Received: by 2002:a4a:d02c:0:0:0:0:0 with SMTP id w12csp431243oor;
Fri, 1 May 2020 02:26:52 -0700 (PDT)
X-Google-Smtp-Source: APiQypJnoy24xkbmtQW186il9sN1ceez2VY4UJdyoFJoFX5Wq6zO6SzHbC6yWt91x0hxhO+zLym8
X-Received: by 2002:a17:906:bcec:: with SMTP id op12mr2356056ejb.245.1588325212696;
Fri, 01 May 2020 02:26:52 -0700 (PDT)
ARC-Seal: i=2; a=rsa-sha256; t=1588325212; cv=pass;
d=google.com; s=arc-20160816;
b=UipsYlthXOWZ0Pw1W9G7jlCR9VV8Okc0el4w6Sd+GtE/0iV8bSmwpGrVMR/yde6v3s
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TVPQ==
ARC-Message-Signature: i=2; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816;
h=mime-version:content-language:accept-language:message-id:date
:thread-index:thread-topic:subject:to:from:dkim-signature;
bh=ZIPFY26fxJfLG0x25BtWrm246txMIPQR1S7hRV8tBRY=;
b=Sr7ILZcU6YpJlmffMa0BCFyXHA2jvHrbM8wpXyHdu3S/s4/NpcErPyj1IBXfZzdjwp
thbdOCW6G2w4OPd57/2RhPNzbTm/OErJuGt84TbJ6QJuGVQcpjp6TRid9nRyCDi3jOhW
uwCXiAmY/jj47os4H5iMzm68WmF9t7aDU3g1W3HpQcA+Xn87MiCIVsqPzOkgzVGhm7sC
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BuKvck7tGBz3BAiI9wtWDEyIcSH/xf2jixm0PeqKOwPN47kL93kzsj4TECSqbQTKwPtx
/QNA==
ARC-Authentication-Results: i=2; mx.google.com;
dkim=pass header.i=@hotmail.com header.s=selector1 header.b=owYkIFWf;
arc=pass (i=1);
spf=pass (google.com: domain of jesenice@hotmail.com designates 40.92.73.31 as permitted sender) smtp.mailfrom=jesenice@hotmail.com;
dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=hotmail.com
Return-Path: <jesenice@hotmail.com>
Received: from EUR04-HE1-obe.outbound.protection.outlook.com (mail-oln040092073031.outbound.protection.outlook.com. [40.92.73.31])
by mx.google.com with ESMTPS id rs10si1405036ejb.288.2020.05.01.02.26.52
for <bla.bla@siol.si>
(version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128);
Fri, 01 May 2020 02:26:52 -0700 (PDT)
Received-SPF: pass (google.com: domain of jesenice@hotmail.com designates 40.92.73.31 as permitted sender) client-ip=40.92.73.31;
Authentication-Results: mx.google.com;
dkim=pass header.i=@hotmail.com header.s=selector1 header.b=owYkIFWf;
arc=pass (i=1);
spf=pass (google.com: domain of jesenice@hotmail.com designates 40.92.73.31 as permitted sender) smtp.mailfrom=jesenice@hotmail.com;
dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=hotmail.com
ARC-Seal: i=1; a=rsa-sha256; s=arcselector9901; d=microsoft.com; cv=none;
b=M8VEPynm3sH5L1KXoKbX28bS0xk+FT0iewCuV7V9LNNSOPHVSqdH5mvwiY3V++LAZQ2ukmkq0h87tePRtZ+90kKJmWflv4RJW19io+up48vKxW0viDyoJak5YuIglkuOajgBm+ilNYkEhXuTJ8wwRD5aEqAT4gosDODvTzixlZyct2Zp9nN+QrwBddxUr/bA0gCwoX6UPBgDecQ6IAkovRTFYQjtc29ND6FC6qqmI8m8FSBnH40GFMWI+/AkpYxfL2sP9lT3Nc9HcD7z0m+6EYBeO6hhXjUTNJwYTxiphoUMpMeZnUMiVzhdIhbDkqVp/4nx/JHMUSPX895gZh9aCg==
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=microsoft.com;
s=arcselector9901;
h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck;
bh=ZIPFY26fxJfLG0x25BtWrm246txMIPQR1S7hRV8tBRY=;
b=HYV5FGVlCLT4pKRpgSvA51D6v726TER9S8gGzlmAKGUNxcYuMmiQfmMhLUvszsE1cVUYUACJbZcXkkwrrAYmKLchpase8ZVIF7UG9bWglibdT22Spd0lCzZ/J9bsXL8FTdZX9blHlUMWESVzSo/mwm65X+6bchtZnfsbrlvpfef2LYbIiyeL8JTiD2YGvlEn4Wcvzs7ETlY3H/JDsYO8abpesYSAz8iUtHYIYk2nWHqGo1hMHPqvXsL9fZZOvyriQE09BaDMhAzlDn2qKABnXEKlfGoEMNFc6dcBwPHRbeOXUnViyS0kpsDMLlHmXtkQY7ARwwKvd1NexdTw8w/mXA==
ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=none; dmarc=none;
dkim=none; arc=none
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=hotmail.com;
s=selector1;
h=From:Date:Subject:Message-ID:Content-Type:MIME-Version:X-MS-Exchange-SenderADCheck;
bh=ZIPFY26fxJfLG0x25BtWrm246txMIPQR1S7hRV8tBRY=;
b=owYkIFWfCC1NRzcBToTlVZie3ltYSXG/mLtdn0xvufDwJJtC8Yfsi7xi6+4zmj23IQJv8yqlPaiAOlFSNYKj4e6fyXCNIu7qZakTdATdSrKgkKRXtTxkb+Upf2vxZOVXzEVpMmy2cCxvpkjNT1U7YV1UFTU5vaWEljPQqjQY3Y/DemJkCBMe5iMTsyn0NbY2lI7L/5BIcoCHMTgGUSaT2IymveTFdu2TlGILKrm4hs9HvNHYJeTk3ge7M81UGkEDmG79AGf+46UngfuGbJ3uR6f/a0l5AKp8ZcMblc0kozozaHc5bwm82vW4u2YBKfgCMUnLIO5/qt3DPD868k+UUg==
Received: from VI1EUR04FT019.eop-eur04.prod.protection.outlook.com
(2a01:111:e400:7e0e::52) by
VI1EUR04HT146.eop-eur04.prod.protection.outlook.com (2a01:111:e400:7e0e::394)
with Microsoft SMTP Server (version=TLS1_2,
cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2937.15; Fri, 1 May
2020 09:26:51 +0000
Received: from DBBPR03MB5222.eurprd03.prod.outlook.com
(2a01:111:e400:7e0e::48) by VI1EUR04FT019.mail.protection.outlook.com
(2a01:111:e400:7e0e::251) with Microsoft SMTP Server (version=TLS1_2,
cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2958.19 via Frontend
Transport; Fri, 1 May 2020 09:26:51 +0000
Received: from DBBPR03MB5222.eurprd03.prod.outlook.com
([fe80::8430:b96b:beae:6811]) by DBBPR03MB5222.eurprd03.prod.outlook.com
([fe80::8430:b96b:beae:6811%3]) with mapi id 15.20.2937.028; Fri, 1 May 2020
09:26:51 +0000
From: zajc cjaz <jesenice@hotmail.com>
To: "bla.bla@siol.si" <bla.bla@siol.si>
Subject: =?iso-8859-2?B?qXVtbmlraSDIxq6p0Ojmvrnw?=
Thread-Topic: =?iso-8859-2?B?qXVtbmlraSDIxq6p0Ojmvrnw?=
Thread-Index: AQHWH5qlBsvWxopuUE6OanryTKR1zA==
Date: Fri, 1 May 2020 09:26:51 +0000
Message-ID:
<DBBPR03MB5222E1EFE0BBF05A9A724AF2D7AB0@DBBPR03MB5222.eurprd03.prod.outlook.com>
Accept-Language: en-GB, en-US
Content-Language: en-GB
X-MS-Has-Attach:
X-MS-TNEF-Correlator:
x-incomingtopheadermarker:
OriginalChecksum:293DEE23BB57AA27C521741D9A874C4FBFE4EADA8ABD8A7BAAF939F072622324;UpperCasedChecksum:87E79A55B3FD235CCC02F02663DB48839B9BAD0E8F3F134E418EC60309C057EE;SizeAsReceived:6694;Count:42
x-tmn: [fJayhbUQ3dY3yeq5Zy83521N/EHHjBuZ]
x-ms-publictraffictype: Email
x-incomingheadercount: 42
x-eopattributedmessage: 0
x-ms-office365-filtering-correlation-id: c10c9868-2167-45f3-f0fc-08d7edb1c814
x-ms-traffictypediagnostic: VI1EUR04HT146:
x-microsoft-antispam: BCL:0;
x-microsoft-antispam-message-info:
FuurofOpTl+bO0+efQgdgTcpS7bUCgCx9D4YzGXMQrS9ySkHpHZgOAyKeWeIIaNvQKIQO2pca/nDcp+RczCyiUjHccLiGwj7p6uqAuTCLxQfrcgVi1Zj9rPTdGBuYz0o4YRud620YN2oMuO/tsMV1gxipxQFjGCeKzJJjXwgv03Ph1ntNldokD11I/B4tlJz7AySZXnwaw3DeT5OP+7OjQ==
x-forefront-antispam-report:
CIP:255.255.255.255;CTRY:;LANG:sl;SCL:0;SRV:;IPV:NLI;SFV:NSPM;H:DBBPR03MB5222.eurprd03.prod.outlook.com;PTR:;CAT:NONE;SFTY:;SFS:;DIR:OUT;SFP:1901;
x-ms-exchange-antispam-messagedata:
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x-ms-exchange-transport-forked: True
Content-Type: multipart/alternative;
boundary="_000_DBBPR03MB5222E1EFE0BBF05A9A724AF2D7AB0DBBPR03MB5222eurp_"
MIME-Version: 1.0
X-OriginatorOrg: hotmail.com
X-MS-Exchange-CrossTenant-RMS-PersistedConsumerOrg: 00000000-0000-0000-0000-000000000000
X-MS-Exchange-CrossTenant-Network-Message-Id: c10c9868-2167-45f3-f0fc-08d7edb1c814
X-MS-Exchange-CrossTenant-rms-persistedconsumerorg: 00000000-0000-0000-0000-000000000000
X-MS-Exchange-CrossTenant-originalarrivaltime: 01 May 2020 09:26:51.7484
(UTC)
X-MS-Exchange-CrossTenant-fromentityheader: Internet
X-MS-Exchange-CrossTenant-id: 84df9e7f-e9f6-40af-b435-aaaaaaaaaaaa
X-MS-Exchange-Transport-CrossTenantHeadersStamped: VI1EUR04HT146
--_000_DBBPR03MB5222E1EFE0BBF05A9A724AF2D7AB0DBBPR03MB5222eurp_
Content-Type: text/plain; charset="iso-8859-2"
Content-Transfer-Encoding: quoted-printable
=A9umniki=C8=C6=AE=A9=D0
--_000_DBBPR03MB5222E1EFE0BBF05A9A724AF2D7AB0DBBPR03MB5222eurp_
Content-Type: text/html; charset="iso-8859-2"
Content-Transfer-Encoding: quoted-printable
<html>
<head>
<meta http-equiv=3D"Content-Type" content=3D"text/html; charset=3Diso-8859-=
2">
<style type=3D"text/css" style=3D"display:none;"> P {margin-top:0;margin-bo=
ttom:0;} </style>
</head>
<body dir=3D"ltr">
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
color: rgb(0, 0, 0); background-color: rgb(255, 255, 255);">
=A9umniki=C8=C6=AE=A9=D0</div>
</body>
</html>
--_000_DBBPR03MB5222E1EFE0BBF05A9A724AF2D7AB0DBBPR03MB5222eurp_--