internet security warning

[#1]

When sending mail from my main Outlook address I get the warning about certificate cannot be verified. I enclose copies. Viewing the certificate I can't see anything wrong in the "target name" if that is relay.plus.net and the date of validity is also correct. I've no idea whether the identifying numbers are correct. What do I do - currently I've just been continuing to use the 

[#2]

Moderator's Note

This topic has been moved from Broadband to Email

[#3]

are you sending e mails with a digital signature; it's where the problem may be.

In Outlook click FILE, then

 OPTIONS,

TRUST CENTRE,

E Mail Security, unclick "Add digital signature", OK

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To check your certificates on a Windows machine

Click "windows key" + R

enter "certmgr.msc" [no quote marks]

Click on PERSONAL

open CERTIFICATES

Are any in the list expired?

That'll be issue.

Either delete, or replace with a fresh one.

[#4]

Thank you for your response. I'm not using a digital signature. There was one out of date cert (applicable to Apple) which I deleted. The long names for   "issued by" do not correspond exactly with the issuer in the "faulty" cert.  But thanks for your interest. 

[#5]

This issue usually arises where OUTLOOK 2019 or 365 "guesses" the SMTP server to be one of...

• smtp.accountname.plus.com
• smtp.yourdomainname.co.uk
• smtp.plus.net

Most certainly the latter will work if you click "accept certificate", however that this not the name in the certificate - the SMTP server should be (as you mentioned) relay.plus.net

[#6]

Interesting info. I don't actually get the invitation to accept but I have a) "continued to use the cert" and b) "installed certificate" - the very one I've been complaining about, I assume. It gets my email out but doesn't solve the underlying fault because it re-appears every time I fire up Outlook.
I thought I'd sent screenshots of the message with my initial  but they don't appear to have gone so I'm trying again with pdf extension. Thanks for your interest and if you can add anything I would be very grateful.

[#7 · Accepted solution]

Is this on sending or on first opening when Outlook updates the mailboxes?

Servers should be:

• Incoming
  • POP3 mail.plus.net
  • IMAP imap.plus.net
• Outgoing SMTP relay.plus.net

Other names might work, but will give rise to certificate warnings.

[#8]

I checked the servers details again and, to my surprise the outgoing was no longer "relay". I have now changed it back to relay and hey-presto. Very many thanks to all who took an interest and especially to Townman - superuser.