Cannot send emails following password change

[#1]

Hi, I wonder if you can assist?

I have a PlusNet email account and run a number of additional or secondary mailboxes. I updated the passwords on a couple of those additional mailboxes and since then I have not been able to send emails from each of those updated mailboxes on most of my devices. However webmail WILL send emails from those mailboxes.

The devices affected are Outlook on the desktop & laptop, two Android phones using Gmail and a new iPad: Oddly the 10-yr old iPad will send emails from those mailboxes.

The settings I am using are:

Username: (my account user name plus mailbox name)
Password: the new mailbox passwords (which vary from mailbox to mailbox)

POP3 Incoming server: mail.plus.net
POP3 Incoming server port: 110

SMTP Outgoing mail server: relay.plus.net
SMTP Outgoing mail server port: 25 (587 also tried)

SSL off for both incoming and outgoing

I have tried changing the passwords again; resetting the router; testing the phones on 4G only (ie remote from all operative WiFi networks). I've tried a number of different port combinations, but nothing seems to work. 

The new iPad comes up with the following message when trying to send emails:

"Cannot Send Mail
The server rejected one of the recipient's addresses. The message has been placed in your Outbox."

Outlook throws up a dialogue box requesting the user name and password (which are already filled in and, as far as I can see, are correct.

Finally on Android, when trying to make changes to the Outgoing settings (eg port number from 587 to 25), produces this message:
"Problem with account setup
Your username or password is incorrect". (even though they are correct)

Please help!

[#2]

You probably have authentication setup on the outgoing server and haven't changed the pasword there

[#3]

Thanks MisterW and my apologies for appearing dumb, but how would I change the relevant password? (I thought I'd changed all the necessary passwords.)

[#4]

but how would I change the relevant password?

Im not familiar with Outlook, but somewhere in the configuration for the outgoing server(relay.plus.net) you have authentication details set of username and password. Thie password here will need amending to the changed one.

[#5]

Thanks MisterW for your suggestion.

This morning I tried another tack: I changed the password for a secondary mailbox I haven't used recently and tried to add the account to my Android phone. I've done this many times in the past without a problem. This time it accepted the incoming server details without a problem, but when I entered the outgoing server details (relay.plus.net) it rejected the outgoing server settings with the message "Your username or password is incorrect". This is despite the username and password being exactly the same as the incoming email server and matching the details I had entered previously.

I then deselected "Require sign-in" which allowed me to complete the set up process. Checking the Inbox revealed that I'd received a welcome email from PlusNet. But still when I tried to send an email it went to the Outbox and was flagged as "Failed".