On pc Debian x86-64 with master sources updated today, I don't reproduce this.

I gave a try with:
Sub Main
	eMailer = createUnoService("com.sun.star.system.SimpleCommandMail")
	eMessage = eMailer.createSimpleMailMessage()
	eMessage.Recipient = "test@test.fr"
	eMessage.Subject="subject"
	eMessage.Body="body1"  + chr$(10) + "body2"  + chr$(10) + "body3"
	eMessage.Attachement=(Array(convertToUrl(filepath)))
	eMailer.sendSimpleMailMessage ( eMessage, _
	com.sun.star.system.SimpleMailClientFlags.NO_USER_INTERFACE )
End Sub

6.0 is an old branch.
Could you give a try to last stable LO version 6.4.5?

Created attachment 164153 [details]
Screenshot of output

Thank you for your feedback.

Could you also attach the precise macro code you used?

The screenshot was with LO 6.4.5

code:

sub send_email(filepath as String, eMailAdress as String, yourOrderno as String)

	Dim eSubject as String
	Dim eMailer as Object
	Dim eMailClient as Object
	Dim eMessage as Object

	eSubject = "Delivery note your order " & yourOrderno

	eMailer = createUnoService("com.sun.star.system.SimpleCommandMail")
	eMessage = eMailer.createSimpleMailMessage()
	eMessage.Recipient = eMailAdress
	eMessage.Subject=eSubject
	eMessage.Body="Text1" & chr$(10) & "Text2" & chr$(10) & "Text3"
	eMessage.Attachement=(Array(convertToUrl(filepath)))
	eMailer.sendSimpleMailMessage ( eMessage, _
	com.sun.star.system.SimpleMailClientFlags.NO_USER_INTERFACE )

End sub

I still don't reproduce the problem with your code (after having args of the function since LO complains I call the macro without args).
It seems I can't help here => uncc myself.
Since I don't have more questions, I'll put it back to UNCONFIRMED.

LO 5.1.6.2 seems to be the last version working.
LO 5.2.7.2 and onwards has this issue.

Tested with claws-mail instead of thunderbird and that was working with LO 6.4.5.

The issue seems to be between thunderbird and LO from version 5.2.7.2 and onwards.

Hello Zeke,
A new major release of LibreOffice is available since this bug was reported.
Could you please try to reproduce it with the latest version of LibreOffice
from https://www.libreoffice.org/download/libreoffice-fresh/ ?
I have set the bug's status to 'NEEDINFO'. Please change it back to
'UNCONFIRMED' if the bug is still present in the latest version.

New test with LO7.

New clean install of Linux Mint 20.1 Cinnamon.

sudo add-apt-repository ppa:libreoffice/ppa

Upgrade LO to latest fresh.

Assign an account to Thunderbird and test it so it can send email.

Run this code:

sub send_email(filepath as String, eMailAdress as String, yourOrderno as String)

Dim eSubject as String
Dim eMailer as Object
Dim eMailClient as Object
Dim eMessage as Object
Dim bodytxtstring as string

eSubject = "Delivery note your order " & yourOrderno
bodytxtstring="Please find delivery note attached" & chr$(10) & "as pdf." & chr$(10) & "Best Regards ..."

eMailer = createUnoService("com.sun.star.system.SimpleCommandMail")
eMessage = eMailer.createSimpleMailMessage()
eMessage.Recipient = eMailAdress
eMessage.Subject=eSubject
eMessage.Body=bodytxtstring
eMessage.Attachement=(Array(convertToUrl(filepath)))
eMailer.sendSimpleMailMessage ( eMessage, _
com.sun.star.system.SimpleMailClientFlags.NO_USER_INTERFACE )

end sub

Result in Thunderbird:

From: zxc@gmail.com
To: vbn@gmail.com
To: Please find delivery note attached
To: as pdf.
To: Best Regards ...
Subject: Delivery note your order 254456
Bodytext: (Nothing)

The bug seems to still be present in LO7.

Due to the bug we use the following code instead:

eSubject = "Delivery note " & deliveryID & " from your order no " & yourOrderno
shellstring="thunderbird -compose subject=" & eSubject & ",to=" & eMailAdress & ",message=/home/user1/Businessystem/txt/Deliveryconftxt,attachment='" & filepath & "'"
shell shellstring

I am also having this EXACT problem with Libreoffice + Linux Mint + Thunderbird setting.

But it works well with evolution or Geary email client.


BTW Zeke, can you elaborate on your workaround code ?

--------------------
Due to the bug we use the following code instead:

eSubject = "Delivery note " & deliveryID & " from your order no " & yourOrderno
shellstring="thunderbird -compose subject=" & eSubject & ",to=" & eMailAdress & ",message=/home/user1/Businessystem/txt/Deliveryconftxt,attachment='" & filepath & "'"
shell shellstring
---------------------  

what exactly is this shellstring stuff ?

Explanation of the workaround we use as requested by Pharmankur:

eSubject = "Delivery note " & deliveryID & " from your order no " & yourOrderno
shellstring="thunderbird -compose subject=" & eSubject & ",to=" & eMailAdress & ",message=/home/user1/Businessystem/txt/Deliveryconftxt,attachment='" & filepath & "'"
shell shellstring

Shell execute a command at the cl (commandline)

Shellstring is a textstring with the command.

thunderbird starts thunderbird.
-compose tells thunderbird to initiate a mail to send.
subject= sets the mails subject.
to= sets the mails reciepent.
message= path to a textfil containing the bodytext of the mail.
attachment= path to a attachment file.

Thank You Zeke for your quick response
That's a great workaround.
Thanks for sharing.

Dear Zeke,
Based on your idea (of using shell script from libreoffice Macro) prepared a demo file.
It makes use of MUTT 

Details are here ---
https://forum.openoffice.org/en/forum/viewtopic.php?f=20&t=90100#p521622

Setting to new as per comment 10.
Version and regression keywords as per comment 6.

Is this the same issue as https://bugzilla.mozilla.org/show_bug.cgi?id=208230 ?
Which would make it "not our bug"?

You must always have one recipient in the 'TO' field for the mass email to be sent. To accomplish this, just enter your own email address in that field.