Which port are you using, Peter?

The failure will happen if

Your Plesk have same domain that is your SMTP host on IONOS

Plesk trying to connect to the port on itself or same server as plesk installed. So it struck in loop.

The relationship between email and nameservers is that when there is confusion over where the nameservers are pointed, then the email may not work, especially if the current nameservers do not hold the correct records. This was the situation in this case.

Thank you for pointing me at the SMTP component, of which I was unaware. In following the points you described, point 4 works perfectly. However, I am experiencing a 504 error in trying to configure the SMTP component at point 6.

I am contacting openteknik.

Sorry, but I can't follow. Your topic is addressing a mailing issue - and now we're talking about changing nameservers!?

If I were a Ionos customer I would:
1. create a Ionos mail account
2. Use a known to work mail client like Outlook or Thunderbird
3. and configure that client to connect to the Ionos smtp server for sending outgoing mails, see
https://www.ionos.com/help/email/general-topics/settings-for-your-email-programs-imap-pop3/
4. Then use that client to send a testmail to a gmail account of mine.

In case I receive that testmail without issues in my gmail box i would
5. install Ossn's SMTP component, see
https://github.com/opensource-socialnetwork/SMTP
6. and configure it accordingly to step 3
7. use my site's 'Invite friends' feature with the same gmail recipient's address as before
8. check my gmail inbox again if that invitation has been received

In case step 4 fails I would contact Ionos
In case step 4 is working flawlessly but step 7 fails I would contact openteknik.com

In transferring the domain to IONOS, it solved the spf record issue, so I thought that solved the problem, and other email providers were happy.,but I find that the DMARK record was not showing, which caused the Gmail problem.

Actually,in doing further research and speaking to my hosting provider (Ionos), I discover that Ionos set up domains to use their own nameservers by default, rather than the Plesk nameservers on the domain! Hence the DNS records I set up on the server did not apply and hence the issues.

We have changed nameservers to be the Plesk nameservers and I await propagation. I hope that anyone using Ionos is now aware of this.

I am. Details posted where requested.

Four months ago you wrote:

"I have found a previous discussion regarding gmail. I note that using
mxtools.com, my spf recod has not been propagated, nor is the PTR
record set. My provider (name.com) says they do not do this because
they only provide shared hosting and PTR records are only relevant for
VPS and other server solutions. They did not advertise the fact when I
bought the domain! So I am tied there for 60 days!"

And you fixed this in the meantime, but still having gmail issues?

Please provide some more details about the site you're talking about
see https://www.opensource-socialnetwork.org/wiki/view/1101/how-to-report-a-bug-error-issue