Thank you for your response.
I have seen that the firewall blocks traffic ICMP-Typ 3 and 4 when I send a mail in this account. Then I have enabled traffic ICMP-Typ 3 and 4 in the firewall.
> 00:43:34.828 7: Socket read error 2746.
> 00:43:43.906 << 0006 QUIT\0D\0A
> 00:43:43.906 9: Socket write error 2746.
was solved. I had not changed anything other (MPU, ...)
As well large mails can now be send in this account without any problems.