different smtp servers for different mail accounts on one iOS device

I have two mail accounts on my devices, my personal account, and my work Exchange account. Each account is configured with its own SMTP server, and that has always been done when the accounts are added to the device. Each account asks for server information, both incoming and SMTP. I've been using the iPhone from the beginning, and the only time I've had to change anything was when Comcast moved from POP to IMAP. As I re-read your posts, I now see that you stated you used the same SMTP settings for each account, so that makes sense that it is a problem.

The arrangement in iOS/iPadOS Settings for the outgoing SMTP server, where you have multiple accounts that use the same server, is (and never has been) particularly clear or intuitive. That said, when you set-up an account, each account has its own unique account settings.

When drilling though the email account settings - and you happen across the list of SMTP servers - you are presented with a list of SMTP servers for which all accounts on the device are provisioned. If you stray from the account-default (created when initially setting-up the account on the device) by choosing an alternative from the presented list, you may create a problem. Most email providers do not allow you to use their SMTP server as a send-relay for other mail providers - and will only accept outgoing mail for their own domain(s).

Generally, unless you understand exactly what you are doing when straying from the created account default servers, I recommend that you avoid manual selection of SMTP server from the list. This ensures that when setting-up an email account on iOS/iPadOS devices, the correct outgoing SMTP server is selected for each specific account.

One question: Are you connecting to your employer over a VPN connection? If so, whilst the VPN is active, any applied network policies applied by the VPN Gateway/Network to which you connect may restrict traffic to policy-defined servers/services.

Okay. In short - I recommend that you don’t attempt to select an alternative SMTP server from the list. Unless you know what you’re doing, this will likely compound any problems you have.

For simplicity, I suggest that you delete all email accounts from your iPad. Assuming that you are using IMAP mailboxes (or Exchange mailboxes if connecting to Corporate email) no mail messages will be lost; all email is retained on the mail server(s).

After deleting the email accounts from your iPad, force-restart the iPad:

• On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
• On an iPad with Face ID: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.

After the restart, add and verify each email account that you need to use - only one at a time using, checking for normal connectivity before moving on to the next account. For each account, accept default server settings.

This will hopefully resolve your problem

Your description would appear to confirm my earlier comments concerning SMTP - and the limitations imposed by email providers designed to inhibit SMTP-relay abuse.

As for automated switching between SMTP servers, outside of enterprise applications, this has limited usefulness. Most email operators facilitate switching between servers, for high availability applications, via DNS. As such, if a server needs to be taken offline for maintenance or other purposes, DNS resolution takes care of (a) switching email clients from one server to another, and (b) load sharing between servers.

Anyhow, it’s good to hear that diagnostics have seemingly resolved your issue.

whether or not it is actually possible to 'switch' between servers (as you can on macOS

To add my two cents: you have to remember that iOS is an over-simplified version of Mac OS, so it does not have the full range of capabilities that Mac OS has.

I fully realise that your problem is not about wanting to explicitly use SMTP-relay (but is evidently what you have been doing). My intended point, perhaps missed, is that the vast majority of email providers will not allow you to send email via their servers for email addresses that do not belong to their domain. This measure will likely impact your configuration.

This measure is implemented, by email providers, to inhibit SMTP-relay abuse - such abuse having became a common problem some years ago.