Comment 28 for bug 1748895

With respect, I think the problem is related to apparmor.
When Tools/Options/Internet/Email is set to /usr/bin/claws-mail, the error window appears (working email client not found), an error message is sent to screen (Writer called from the command line),
"/usr/lib/libreoffice/program/senddoc: 366: /usr/lib/libreoffice/program/senddoc: /usr/bin/claws-mail: Permission denied", and the kernel log contains 3 DENIED lines,
Sep 20 10:09:04 USER_ID-Inspiron-5767 kernel: [ 1965.191418] audit: type=1400 audit(1537434544.991:41): apparmor="DENIED" operation="file_inherit" profile="libreoffice-senddoc" name="/home/USR/errors_090715.odt" pid=4934 comm="senddoc" requested_mask="wr" denied_mask="wr" fsuid=1001 ouid=1001
Sep 20 10:09:04 USER_ID-Inspiron-5767 kernel: [ 1965.191423] audit: type=1400 audit(1537434544.991:42): apparmor="DENIED" operation="file_inherit" profile="libreoffice-senddoc" name="/home/USR/.config/libreoffice/4/user/uno_packages/cache/uno_packages.pmap" pid=4934 comm="senddoc" requested_mask="wr" denied_mask="wr" fsuid=1001 ouid=1001
Sep 20 10:09:05 USER_ID-Inspiron-5767 kernel: [ 1965.196286] audit: type=1400 audit(1537434544.995:43): apparmor="DENIED" operation="exec" profile="libreoffice-senddoc" name="/usr/bin/claws-mail" pid=4940 comm="senddoc" requested_mask="x" denied_mask="x" fsuid=1001 ouid=0

If Tools ... Email is set to xdg-email (timed at 10:12) or set to be blank (timed at 10:16), then no error window appears, the screen error message does not appear, and two DENIED lines appear in the kernel log:

Sep 20 10:12:24 USER_ID-Inspiron-5767 kernel: [ 2164.882567] audit: type=1400 audit(1537434744.681:53): apparmor="DENIED" operation="file_inherit" profile="libreoffice-senddoc" name="/home/USR/errors_090715.odt" pid=5014 comm="senddoc" requested_mask="wr" denied_mask="wr" fsuid=1001 ouid=1001
Sep 20 10:12:24 USER_ID-Inspiron-5767 kernel: [ 2164.882572] audit: type=1400 audit(1537434744.681:54): apparmor="DENIED" operation="file_inherit" profile="libreoffice-senddoc" name="/home/USR/.config/libreoffice/4/user/uno_packages/cache/uno_packages.pmap" pid=5014 comm="senddoc" requested_mask="wr" denied_mask="wr" fsuid=1001 ouid=1001

Sep 20 10:16:27 USER_ID-Inspiron-5767 kernel: [ 2408.161860] audit: type=1400 audit(1537434987.959:65): apparmor="DENIED" operation="file_inherit" profile="libreoffice-senddoc" name="/home/USR/errors_090715.odt" pid=5171 comm="senddoc" requested_mask="wr" denied_mask="wr" fsuid=1001 ouid=1001
Sep 20 10:16:27 USER_ID-Inspiron-5767 kernel: [ 2408.161865] audit: type=1400 audit(1537434987.959:66): apparmor="DENIED" operation="file_inherit" profile="libreoffice-senddoc" name="/home/USR/.config/libreoffice/4/user/uno_packages/cache/uno_packages.pmap" pid=5171 comm="senddoc" requested_mask="wr" denied_mask="wr" fsuid=1001 ouid=1001