Outlook and Gmail problems from PC

[#1]

Can’t get my head around this, perhaps someone could give me a pointer.

Emails from my iPad or iPhone to a Gmail or Outlook address are delivered normally. But the same emails sent from my PC (using Outlook client) are bounced by Gmail with an authentication error and the ones to Outlook aren’t delivered but without any error response. Up to now they’ve been fine and I’ve not changed anything (aside from the usual Windows updates).

The account details are the same on all my devices (relay and imap at force9) and they’ve not been changed for umpteen years with no problems up to now. Emails to other recipients including my own Force9 address seem to arrive OK.

Any email expert got a suggestion to follow up? Thanks.

[#2]

There are certainly reports of issues in sending to Microsoft email domains from *.plus.com using the Outlook client, where there are multiple addressees.  These give rise to DKIM failures.  The empirical evidence is that sending to the addressees individually is OK as is sending the message as plain text (not HTLM / Rich Text).

Sending to Gmail users might encounter some delays at peak times.

There are extensive threads on each of these issues elsewhere.

Ensure on all devices that the SMTP server connections are configured to authenticate with the same credentials as the inbound server.