I have two domains with full service email both work ok with clients on Windows but have been having problems with IMAP and SMTP on iOS.

I have copied the connection information from the dashboard

IMAP server (my domain with hyphens instead of the dots)-mail.dynu.com port = 993 or 143
SMTP server same as above but SMTP instead of mail port = 587 or 25
Username (username)@domain.co.uk
Password tested with webmail

I thought the problem was with DNS with in my network but have tested it with 4 different connections on 2 Ipads and a Iphone. one of the connections was Virgin media with opnsense as the router, The second was a new FTTH connection I had fitted this week with the provided router, The 3rd and 4th are two mobile networks which I have tethered the Ipads to.

Are there any known issues with Dynu IMAP and iOS?

There is no known issue with Dynu IMAP or SMTP on iOS, and the fact that the same accounts work correctly from your Windows client is useful information - it confirms the mailbox, the credentials and the server side are all fine. The problem is on the device.

What we have seen repeatedly is that the iOS Mail app does not reliably apply changes to an account that already exists. If an account was first added with a setting that was wrong - a server name, a port, or the username - then correcting it in Settings afterwards often makes no difference, because Mail continues to use the values it cached when the account was created. The settings screen shows the new values while the app keeps using the old ones, which makes it look as though the correct settings are being rejected.

This also explains why testing on four different connections and three devices did not change anything. The stale configuration is stored per account on each device, so it travels with the account rather than with the network.

The reliable fix is to remove the account completely and add it again from scratch:

1. On the device, go to Settings, then Apps, then Mail, then Mail Accounts, and delete the account entirely. Editing it is not sufficient - it has to be deleted.
2. Close the Mail app fully, so it is not left running in the background.
3. Restart the device if you are able to.
4. Add the account again, entering the correct settings at the point of creation rather than accepting defaults and correcting them afterwards.

One thing worth checking while you are in there: if you have previously added any SMTP servers manually, iOS keeps them under the account's Outgoing Mail Server screen as "Other SMTP Servers", and these survive the account being deleted. If there are stale entries there, remove them too.

For the settings themselves, please take them from your Dynu control panel rather than from any earlier notes. The incoming and outgoing server names for Full Service Email are specific to your domain, so they are not the same for every account, and the control panel shows the exact values for yours.

There is a step-by-step guide for iOS here:

https://www.dynu.com/en-US/Resources/Tutorials/EmailServices/ClientConfiguration/Set-up-an-email-account-on-your-iPhone-iPad-or-iPod-touch

One last point that may be relevant given how much testing you have already done. If an incorrect password is attempted repeatedly from the same IP address, that address is temporarily blocked for a period as a brute force protection measure. After a large number of failed attempts it is worth leaving it for an hour before trying again, otherwise a correctly configured account can still fail to connect and it looks like the settings are still wrong.

If it still will not connect after a clean re-add, please open a support ticket with the exact error iOS shows and the server names you are entering, and we will take a closer look.