Question Kunena 5.2.6 Email Notification Error

Kunena config is exactly the same as when the email notifications worked. I cannot think about any change to the site that would cause this problem, the last forum email notification was delivered on Wed, 2 Feb 2022 02:11:55 -0800 (PST)

I am repeating the error as provided by my host:

SMTP syntax error in "RCPT TO: [GGC Forum] <forum@>" H=() [127.0.0.1]:52596 I=[127.0.0.1]:25 missing or malformed local part (expected word or "<")
2022-02-08 11:26:57.198 [51155] H=() [127.0.0.1]:52596 I=[127.0.0.1]:25 incomplete transaction (connection lost) from <forum@>

The Kunena debug error is: Could not instantiate mail function.

If I search a bit for the error message I see for example this
stackoverflow.com/questions/10494065/smt...malformed-local-part
so check out the Kunena Forum: Configuration tab users > Subscriptions whether E-mail to Multiple Recipients is set to 0 ?
 

It is set to 10 to limit the number of emails sent from the forum


 

have you tried setting the E-mail to Multiple Recipients to 0?
and is the Visible E-mail Recipient a good E-mail address, so made in your domain environment.
and what happens if you leave it empty does it work.

Thanks for that, yes it works with E-mail to Multiple Recipients = 0, however, this is not an optimal solution because each post to the forum, on this particular website, will generate 100+ email messages, and I am sure that sooner or later this will exceed the number per hour emails set by service provider.
The real issue is why having a higher value as E-mail to Multiple Recipients causes an issue? Is the problem with Kunena or something else?

I have no idea what the more value is of send to multiple recipients.
On my own and also another forum where I also keep up with Kunena as co-administrator,
I have never entered anything there I also had no problems sending too many emails.
Therefore the question to test with the value 0 .
But maybe someone else on this forum knows what the use is of this institution