SMPT user name keeps changeing?
I have 5 email addresses which all receive emails fine. When I set the security and authentication user name on the 1st email account it also changes the user name on the 5th account to the same user name so this account will then not send emails. If I then change the name back on the 5th account to the correct user name this then changes the user name on the 1st account to be the same? The 3 email accounts listed in between are unaffected.
Opaite Mbohovái (1)
It looks like you have the 5th account sending through the smtp server that has the user name of the 1st account. Some services, like google, will automatically switch the sending address to the one associated with the actual smtp server, unless additional steps are made on the gmail website.
Open Tools/Account Settings and select an account in the left pane. Note the selection for Outgoing Server (SMTP) in the lower right pane. Each account should be sending through the smtp server that uses the same user name and password, e.g. gmail account 1 sends through smtp.gmail.com that has the user name and password of gmail account 1, and the same for accounts 2,3,4... You probably have all the necessary servers defined in Tools/Account Settings, Outgoing Server (SMTP) at the bottom of the left pane, but need to make the proper associations between incoming and outgoing servers.