Problems with sending mail. Error "Invalid e-mail address: <email>"

[ank0l0g]

Hello ALL!

In an organization there are two mail domains.
The first "office.com" second "office" Correspondingly mailbox names first server "[email protected]", "[email protected]". Second server "mail1@office", "mail2@office".
Mail runs between them. But after RoundCube, box "[email protected]" no mail is sent to the boxes "mail1@office". Via e-mail client MS poisoned mail.
In this pop up error "Invalid e-mail address: "

Which way to dig?