After upgrading to version 5.4, the message body no longer received a hint about the encoding to the mail client, at the same time the encoding in the subject is correct.

EXAMPLE

From: <[email protected]>

To: <[email protected]>

In-Reply-To: 7788978.84ae30f461ff4dd9ad3f677b73fadb1b.1.82b5dbd743fdd972d01e34de098d8bf2@zabbix.com

Date: Wed, 19 May 2021 07:28:02 +0300

Subject: =?UTF-8?B?VFpCWDogUFJPQkxFTTog0J3QsCDQkdCUINCh0J/QmNChINC00L7Qu9GPINGD?=

=?UTF-8?B?0YHQv9C10YjQvdGL0YUg0LfQsNC/0YDQvtGB0L7QsiDQsiDRgNCw0LfRgNC1?=

=?UTF-8?B?0LfQtSDRg9GB0LvRg9CzIDwgNTAlIFtzcGlzXQ==?=

MIME-Version: 1.0

Content-Type: text/plain

Content-Transfer-Encoding: base64

VHJpZ2dlcjog0J3QsCDQkdCUINCh0J/QmNChINC00L7Qu9GPINGD0YHQv9C10YjQvdGL0YUg0LfQ

sNC/0YDQvtGB0L7QsiDQsiDRgNCw0LfRgNC10LfQtSDRg9GB0LvRg9CzIDwgNTAlDQpUcmlnZ2Vy

IHN0YXR1czogUFJPQkxFTQ0KVHJpZ2dlciBzZXZlcml0eTog0JLRi9GB0L7QutCw0Y8NClRyaWdn

ZXIgVVJMOiANCg0KSXRlbSB2YWx1ZXM6DQoNCjEuINCh0J/QmNChOiDQtNC+0LvRjyDRg9GB0L/Q

tdGI0L3Ri9GFINC30LDQv9GA0L7RgdC+0LIg0LIg0YDQsNC30YDQtdC30LUg0YPRgdC70YPQsyAo

0KHQn9CY0KEg0LHQuNC30L3QtdGBINC80LXRgtGA0LjQutC4OmRiLm9kYmMuc2VsZWN0W1NQSVNf

R09PRF8sU1BJU10pOiDQoNC+0YHRgtC10LvQtdC60L7QvC1PbmxpbWUgIC0gIDMzLjMzJQ0KMi4g

KlVOS05PV04qICgqVU5LTk9XTio6KlVOS05PV04qKTogKlVOS05PV04qDQozLiAqVU5LTk9XTiog

KCpVTktOT1dOKjoqVU5LTk9XTiopOiAqVU5LTk9XTioNCg0KT3JpZ2luYWwgZXZlbnQgSUQ6IDc3

ODg5NzgNCg0KDQrQodC/0LjRgdC+0Log0YPRgdC70YPQsyDRgSDQtNC+0LvQtdC5INGD0YHQv9C1

0YjQvdGL0YUg0LfQsNC/0YDQvtGB0L7Qsiwg0L3Rg9C20L3QviDQv9GA0L7QstC10YDQuNGC0Ywg

0LIg0JDQoNCc0LUg0YDQsNC30LHQvtGA0LAg0LrQvtC90YTQu9C40LrRgtC90YvRhSDRgdC40YLR

g9Cw0YbQuNC5INC4INC/0L4g0YDQtdC30YPQu9GM0YLQsNGC0LDQvCDQsNC90LDQu9C40LfQsCAo

0L7QtNC40L0g0Lgg0YLQvtGCINC20LUg0L3QvtC80LXRgCDQuNC70Lgg0LrQu9C40LXQvdGCINCy

0LHQuNCy0LDQtdGCINC60YDQuNCy0YvQtSDQsNGC0YDQuNCx0YPRgtGLKSDQu9C40LHQviDQt9Cw

0LrRgNGL0YLRjCDRgtGA0LjQs9Cz0LXRgCwg0LvQuNCx0L4g0L3QsNC/0LjRgdCw0YLRjCDQsNCz

0YDQtdCz0LDRgtC+0YDRgy4NCiEhIdCS0J3QmNCc0JDQndCY0JUhISENCtCi0YDQuNCz0LPQtdGA

INC80L7QttC10YIg0YHRgNCw0LHQvtGC0LDRgtGMINCyINGC0LXRh9C10L3QuNC1IDUg0LzQuNC9

0YPRgiDRgdC90L7QstCwLCDQvy7RjS4g0YDQtdC60L7QvNC10L3QtNGD0Y4g0LLRi9C20LTQsNGC

0Ywg0LLRgNC10LzRjw0KaHR0cHM6Ly9vcGVyZGJvMS9zcGlzL3BheXZpZXcuYXNweA==

fixing the content-type header fixes the issue

.