RBR20 router won’t allow access to outgoing Earthlink email server

[Rcot]

Equipment: RBR20 router with 2 satellites. 
ISP: Spectrum

Devices used: two Dell laptops, two ipads, two iPhone 12. All connect via WiFi. Email apps used are both Outlook and the built-in iOS mail app.

Problem: Everything seems to work fine, with one big exception. We cannot send email using the Earthlink smtpauth.earthlink.net server. This is the cas accross the board on all devices. If I turn off WiFi so that I'm using LTE, sending mail works. If I replace the Orbi with a Linksys router, email sends. Reattach the Orbi and email send fails, stating that the server can't be accessed. It appears that the Orbi is blocking that server. 

Can anyone shed any light on cause or fix? My next move would be to nuke the Orbi and set it up again from scratch. Thanks for any help.

[plemans]

since you're using an email program, does it work if you're sending from an online hosted program?

Have you tried a different dns? like googles at 8.8.8.8 or 8.8.4.4   or even cloudfare at 1.1.1.1

[Rcot]

If I understand your question, if I log into the Earthlink web mail service, all is well. Also, if I use iCloud mail, it works. Only Earthlink fails.

[CrimpOn]

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@Rcot wrote:

Devices used: two Dell laptops

Can anyone shed any light on cause or fix? My next move would be to nuke the Orbi and set it up again from scratch. Thanks for any help.

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@plemanssuggested experimenting with using a different DNS server setting on the Orbi. (on the Orbi web interface, Internet page).

Another experiment would be to open a DOS window on one of the Dell laptops and do a nslookup for smtpauth.earthlink.net 

My computer returns this:

Non-authoritative answer:
Name: smtpauth.earthlink.net
Address: 51.81.61.14
Name: smtpauth.earthlink.net
Address: 51.81.61.13
Name: smtpauth.earthlink.net
Address: 51.81.229.172
Name: smtpauth.earthlink.net
Address: 51.81.229.171

Are Armor or Parental Controls enabled on this Orbi?