Malformed subject and message

Hi!

We are happily using Formidable Forms Lite and about to upgrade to a paid license.

However, we are seeing a problem.

When we check Settings > Message > “Prevent plugins and themes from changing this”, the emails are being delivered as HTML and we are seeing this kind of text in the PostSMTP logs in the subject lines for those messages:

=?UTF-8?B?V2VsY29tZSB0byBNYWluZSBDb2FzdCBIZXJpdGFnZSBUcnVzdA==?= ETC

In addition, we get this status error:

5.2.0 STOREDRV.Submission.Exception:SendAsDeniedException.MapiExceptionSendAsDenied; Failed to process message due to a permanent exception with message Cannot submit message. 0.35250:CF050000, 1.36674:0A000000, 1.61250:00000000, 1.45378:02000000, 1.44866:A0290000, 1.36674:0E000000, 1.61250:00000000, 1.45378:A5290000, 1.44866:26010000, 16.55847:FF160000, 17.43559:0000000004020000000000000000000000000000, 20.52176:140F938A1E00000000000000, 20.50032:140F938A8E1770200A00808A, 0.35180:140F938A, 255.23226:ED050000, 255.27962:0A000000, 255.27962:0E000000, 255.31418:F2050000, 0.35250:0A000000, 1.36674:0A000000, 1.61250:00000000, 1.45378:02000000, 1.44866:5A000000, 1.36674:32000000, 1.61250:00000000, 1.45378:5F000000, 1.44866:01000000, 16.55847:C6000000, 17.43559:0000000030030000000000000100000000000000, 20.52176:140F938A1E0010100B060000, 20.50032:140F938A8E17000003000136, 0.35180:10060000, 255.23226:0A00E08A, 255.27962:0A000000, 255.27962:32000000, 255.17082:DC040000, 0.27745:4800D13D, 4.21921:DC040000, 255.279

When we deselect “Prevent plugins and themes from changing this”, the message goes through fine, although the subject shows up as UTF in the PostSMTP logs.

Help?

(Cross-posted with https://wordpress.org/support/?post_type=topic&p=12423013)

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