Question SMTP Change in Joomla Global Config - now subscribers don't get emails

There are various references to mail issues here, none of which ever seem to have a resolution. Mine is specific. We switched yesterday from PHP Mail to SMTP and mail is being sent fine from all other components- but subscription alert emails in Kunena are not being sent.
I have a hunch this has to do with subscribers being sent as 'undisclosed-recipients' which a previous post from years ago ()  seemed to indicate but as usual, no resolution other than to abandon the subscriptions feature. The issue appears to be the SMTP host blocking 'undisclosed recipients' or perhaps placing a limit and stopping it if it exceeds it.
I'll get in touch with my host, but if anyone has specifically experience with SMTP and its related issues with subscription alert emails and has things that have worked, then please get in touch. My config below- i realise I'm not on the latest issue, but this seems to span multiple versions of Kunena, so unlikely to be a resolution.
TYIA

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htaccess: Exists | PHP environment: Max execution time: 30 seconds | Max execution memory: 512M | Max file upload: 2M

Kunena menu details:

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ID	Name	Menutype	Link	Path	In trash
792	Forum	kunenamenu	view=home&defaultmenu=794	forum	No
793	Home	kunenamenu	view=category&layout=list&catid=0	forum/home	No
794	Recent Posts	kunenamenu	view=topics&mode=replies	forum/recent-posts	No
795	Unread Posts	kunenamenu	view=topics&layout=unread	forum/unread-posts	No
796	Compose Post	kunenamenu	view=topic&layout=create	forum/compose-post	No
797	No Replies	kunenamenu	view=topics&mode=noreplies	forum/noreplies	No
798	My Posts	kunenamenu	view=topics&layout=user&mode=default	forum/my-posts	No
799	My Profile	kunenamenu	view=user	forum/my-profile	No
840	Preferences	kunenamenu	view=user&layout=edit	forum/preferences	No
800	Help	kunenamenu	view=misc	forum/help	No
801	Search	kunenamenu	view=search	forum/search	No

Joomla default template details : astroid_template_zero | author: JoomDev | version: 2.2.3 | creationdate: July 2019

Kunena default template details : JF Simone | author: | version: 1.0 | creationdate: 20.05.17

Kunena template params:

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DefaultCategoryicon	icon-folder
profileIconset	default
DefaultIconset	default
editorIconset	default
lightboxColor	white
bootstrap	1
icomoon	0
tooltips	1
borderless	1
SubjectLengthMessage	80
fullactions	1
quick	1
displayMenu	1
displayDropdownMenu	1
displayDropdownContent	1
displayModule	1
displayBreadcrumb	1
displayAnnouncement	1
displayFooter	1
avatarPosition	left
avatarType	img-circle
labels	0
whoisonlineName	default
formRecover	0
topicicontype	fa
fontawesome	0
socialshare	0
socialsharetag
socialtheme	flat
writeaccess	0
localstorage	0
jf_ku_Layout	1
jf_ku_GoogleWebFont	1
jf_ku_GoogleFontSheet	//:300,500,600,700,400
jf_ku_GoogleFontTags	#kunena,#kunena input,#kunena button,#kunena select,#kunena textarea,#
jf_ku_GoogleFontFace	Poppins
jf_ku_Preloader	0
jf_ku_UC_accent_1	#009fe3
jf_ku_UC_text_color	#666666
jf_ku_UC_SideBlock_color	#29235c
jf_ku_UC_tag_color	#4d4d4d
jf_ku_UC_cat_icon_bg	#009fe3
jf_ku_UC_cat_icon_itself	#ffffff
jf_ku_UC_cat_icon_new	#0f9d58
video	1
maps	1
emoticons	1
twitter	1
ebay	1
link	1
picture	1
hide	1
spoiler	1
table	1
code	1
quote	1
divider	1
instagram	1
soundcloud	1
confidential	1
hr	1
listitem	1
supscript	1
subscript	1
numericlist	1
bulletedlist	1
alignright	1
alignleft	1
center	1
underline	1
italic	1
bold	1
strikethrough	1
colors	1
size	1
avatarSizeX	40
avatarSizeY	90
avatarSizeXThumb	36
avatarSizeYThumb	36
avatarSizeXWelcome	72
avatarSizeYWelcome	72
avatarSizeXList	36
avatarSizeYList	36
avatarSizeXPost	144
avatarSizeYPost	144
avatarSizeXProfile	200
avatarSizeYProfile	200
templatebyText
templatebyName
templatebyLink
wysibb	bold,italic,underline,strike,sup,sub,justifyleft,justifycenter,justifyright,|,img,video,link,|,bullist,numlist,|,fontcolor,fontsize,fontfamily,|,quote,code,table,removeFormat

Kunena version detailed: Kunena 5.1.14 | 2019-08-13 [ Nix ]
        | Kunena detailed configuration:

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Kunena config settings:
board_offline	0
enablerss	1
threads_per_page	10
messages_per_page	5
messages_per_page_search	15
showhistory	1
historylimit	6
shownew	1
disemoticons	0
template	jf_simone
showannouncement	1
avataroncat	0
catimagepath	category_images
showchildcaticon	1
rtewidth	450
rteheight	300
enableforumjump	1
reportmsg	0
username	1
askemail	0
showemail	0
showuserstats	0
showkarma	1
useredit	1
useredittime	0
useredittimegrace	600
editmarkup	1
allowsubscriptions	1
subscriptionschecked	0
allowfavorites	1
maxsubject	50
maxsig	300
regonly	0
pubwrite	0
floodprotection	0
mailmod	0
mailadmin	0
captcha	0
mailfull	1
allowavatarupload	1
allowavatargallery	1
avatarquality	75
avatarsize	2048
imageheight	800
imagewidth	800
imagesize	2000
filetypes	txt,rtf,pdf,zip,,tgz,
filesize	2000
showranking	1
rankimages	1
userlist_rows	30
userlist_online	1
userlist_avatar	1
userlist_posts	1
userlist_karma	1
userlist_email	0
userlist_joindate	1
userlist_lastvisitdate	1
userlist_userhits	1
latestcategory
showstats	1
showwhoisonline	1
showgenstats	1
showpopuserstats	1
popusercount	5
showpopsubjectstats	1
popsubjectcount	5
showspoilertag	1
showvideotag	1
showebaytag	1
trimlongurls	1
trimlongurlsfront	40
trimlongurlsback	20
autoembedyoutube	1
autoembedebay	1
ebaylanguagecode	en-us
sessiontimeout	1800
highlightcode	0
rss_type	topic
rss_timelimit	month
rss_limit	100
rss_included_categories
rss_excluded_categories
rss_specification	rss2.0
rss_allow_html	1
rss_author_format	name
rss_author_in_title	1
rss_word_count	0
rss_old_titles	1
rss_cache	900
defaultpage	recent
default_sort	desc
sef	1
showimgforguest	1
showfileforguest	1
pollnboptions	4
pollallowvoteone	1
pollenabled	1
poppollscount	5
showpoppollstats	1
polltimebtvotes	00:15:00
pollnbvotesbyuser	100
pollresultsuserslist	1
allow_user_edit_poll	0
maxpersotext	50
ordering_system	mesid
post_dateformat	ago
post_dateformat_hover	datetime
hide_ip	1
imagetypes	jpg,jpeg,gif,png
checkmimetypes	1
imagemimetypes	image/jpeg,image/jpg,image/gif,image/png
imagequality	50
thumbheight	32
thumbwidth	32
hideuserprofileinfo	put_empty
boxghostmessage	0
userdeletetmessage	0
latestcategory_in	1
topicicons	1
debug	0
catsautosubscribed	0
showbannedreason	1
showthankyou	0
showpopthankyoustats	1
popthankscount	5
mod_see_deleted	0
bbcode_img_secure	text
listcat_show_moderators	1
lightbox	1
show_list_time	720
show_session_type	2
show_session_starttime	1800
userlist_allowed	1
userlist_count_users	1
enable_threaded_layouts	0
category_subscriptions	topic
topic_subscriptions	first
pubprofile	1
thankyou_max	1
email_recipient_count	0
email_recipient_privacy	bcc
captcha_post_limit	0
image_upload	registered
file_upload	registered
topic_layout	flat
time_to_create_page	1
show_imgfiles_manage_profile	1
hold_newusers_posts	0
hold_guest_posts	0
attachment_limit	8
pickup_category	0
article_display	intro
send_emails	1
fallback_english	1
cache	1
cache_time	60
iptracking	1
rss_feedburner_url
autolink	1
access_component	1
statslink_allowed	1
superadmin_userlist	0
legacy_urls	0
attachment_protection	0
categoryicons	1
avatarresizemethod	1
avatarcrop	0
user_report	0
searchtime	365
teaser	1
ebay_language	0
allow_change_subject	0
max_links	6
read_only	0
ratingenabled	0
url_subject_topic	0
log_moderation	0
attach_start	0
attach_end	14
attachment_utf8	1
autoembedsoundcloud	1
emailheader	media/kunena/email/
user_status	1
signature	1
personal	1
social	1
plain_email	0
moderator_permdelete	0
avatartypes	gif, jpeg, jpg, png
smartlinking	0
defaultavatar
defaultavatarsmall	s_
quickreply	0
avataredit	0
activemenuitem
mainmenu_id
home_id
index_id
moderators_id
topiclist_id
misc_id
profile_id
search_id
avatar_type	0
sef_redirect	1
allow_edit_poll	1
use_system_emails	0
autoembedinstagram	1
disable_re	1

| Kunena integration settings:

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Kunena - Finder Disabled
 Kunena - AltaUserPoints Disabled
 Kunena - Community Builder Disabled
 Kunena - Easyblog Disabled
 Kunena - Easyprofile Disabled
 Kunena - Easysocial Disabled
 Kunena - Gravatar Disabled
 Kunena - JomSocial Disabled
 Kunena - Joomla Enabled:  access=1  login=1  
 Kunena - Kunena Enabled:  avatar=1  profile=1  
 Kunena - UddeIM Enabled:  private=1  

| Joomla! detailed language files installed:

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Joomla! languages installed:
en-GB	English (United Kingdom)

Third-party components: UddeIM 4.0

Third-party SEF components: None

Plugins: None

Modules: None