Unable to send emails

[#1]

I'm suddenly getting messages form Outlook when I try to send emails that my login is being rejected by the server. I haven't changed anything at my end except my internet provider, which shouldn't make any difference. Is there anything I can do here? I've tried repairing my Outlook profile and updating all information it it but don't seem to be getting anywhere.

MTIA

Rob

[#2]

I haven't changed anything at my end except my internet provider, which shouldn't make any difference.

Changed from which provider to which ?

and which outgoing server are you using to send email ?

[#3]

From BT to using mobile broadband  whilst waiting for the full fibre cables to be connect for a non-Openreach provider. I've been using the mobile broadband for two weeks now. following a 5G change locally I had to put my router back to factory settings then change the settings. APN has been changed. 

Servers are mail.plus.net for incoming on port 995 (ssl/tls) , relay.plus.net for outgoing. has been on port 25 SPA enabled. 

I've tried every combination of ports (587 and 465) with SSL/TLS or STARTTLS. Using encryption returns an error message

"Send test email message: None of the authentication methods supported by this client are supported by your server." 

I tried logging on to Webmail and initially got an authentication error there while sending mail but that passed after about 30 minutes.  I've reset my passwords on the mail server and am at a bit of a loss now.

[#4]

I've tried every combination of ports (587 and 465) with SSL/TLS or STARTTLS

The correct one to use is port 587 with STARTTLS

However that's likely not the problem. Using mobile broadband, public IP addresses are shared between many users using CGNAT. It's possible that the IP address you're using has been given a bad reputation. Problem is that many email clients dont report the actual error, rather just an authentication error. If it is a bad reputation then it will likely sort itself after 24 hours.

[#5]

Having occasional issues with authentication sending from webmail. It seems after a few rejections of the Outlook attempts it wont send from Webmail either.

[#6]

I believe too many rejected attempts will temporarily prevent access to the server for a period

[#7]

Try powering down the mobile router for an hour, the reconnect to see if you get a different IP address.

[#8 · Accepted solution]

I use static IP addresses for my desktops (Windows, Mac and Raspberry Pi’s) and WiFi extenders, DHCP for everything else.

Whatever was wrong now seems to have fixed itself as this was sent from Outlook.