At this point, although you may have various smtp servers, it's not clear that each account is actually set up to use their own smtp server.

Please post some info: Help > Troubleshooting Information Under 'Mail and News Accounts' Please copy all the information and post it into this forum.

How to ensure each mail account is using it's own SMTP server.

Right click on the first email account name folder in Folder Pane and select 'Settings' to open Account Settings window. Scroll to bottom in left pane and select 'Outgoing Server (SMTP)' this will show you all the outgoing servers listed. Select each in turn Click on 'Edit' 'Description '- enter the email address which should be set as the 'User Name' = full email address. click on OK

Then repeat working down through list of smtp servers, ensuring the Username email is entered into the 'Description' field.

At this point, you should have a list of smtp servers which shows one smtp per email address which is now more clearly visible after editing the 'Description field.

Now make sure each mail account is set up to use the correct outgoing SMTP specifically for that accounts email address. In left pane select the first mail account name. On the right side, at the bottom, you will see 'Outgoing server (SMTP)' Does it say the same email address (which you just put into the Description field) as the account name you selected? If no, then click on the drop down option (do not click on Edit SMTP button) and select the correct outgoing server which has the correct email address as the account.

Repeat for each mail account listed in left pane. Checking the SMTP it is using.

See images below as guide. In image below - I have two accounts that use identical server info, so I changed the Description in one of them and then made sure that the account was actually using the correct smtp server for that account.

Thanks, that's solved my issue.

It wasn't clear to me that I had to set up the association from the account name (at the top).