Mail SMTP configuration error: how to solve?

I have two problems with configuring SMTP accounts on my iPhone (4s, with IOS7):

1) the SMTP is configured, but iOS puts the wrong username together, so login fails. Here is, how:

SMTP server: smtp.myprovider.com (this is the SMTP of my provider)

username: claus@mydomain.com (mydomain.com is the domain I host there, and the EMAIL is my username!)

All other settings are trivial and correct.

... And now my iPhone connects (correctly) to the SMTP smtp.somewhere.com, but logs in with the username "claus@mydomain.com@smtp.myprovider.com" which obviously must fail.

Any suggestions?

2) I have several accounts (POP and IMAP), all of which need THEIR SPECIFIC SMTP account. Why?

Because an increasing number of providers have their SPAM filters configured so as they throw away (they don't even reject sometimes) all mail which is not sent by the SMTP of the provider where the domain is hosted (quite sensibly, though).

iOS lets me configure a primary SMTP and other SMTP, but does not let me configure ONE SMTP per ACCOUNT each. Or did I miss something?

The result: All mail is sent via the primary SMTP (which accepts mails from any email to any email as long as I am authentified), and some mails get thrown away by SPAM filters because the SMTP does not match the mail address.

Any suggestions would be deeply appreciated!

These things just threw my iPhone useless for mail for any mail account with is (what a shame!) not a MS Exchange account ... Steve would rotate in his grave if he heared that.