[horde] only RSET accepted now

[Twilek]

Hi all,

I use the latest Horde installation via pear with PHP7 and the latest  
exim as an IMAP / SMTP server. Since about 2 month I get an error when  
trying to send mails with a larger attachment (I don´t know exactly,  
but it seems to be around 1 MB). The error reads:

  2016-12-18T18:40:08+00:00 ERR: HORDE [imp] only RSET accepted now  
only RSET accepted now [pid 32431 on line 1160 of  
"/var/www/webmail/imp/lib/Compose.php"]
  2016-12-18T18:42:23+00:00 ERR: HORDE [imp] only RSET accepted now  
only RSET accepted now [pid 3074 on line 1160 of  
"/var/www/webmail/imp/lib/Compose.php"]
  2016-12-18T18:42:23+00:00 WARN: HORDE [imp] PHP ERROR: fwrite():  
SSL: Die Verbindung wurde vom Kommunikationspartner zurückgesetzt [pid  
3074 on line 92 of "/usr/share/php/Horde/Smtp/Connection.php"]

Mails without or with small attachments work flawlessly. Googling  
turned up nothing definite but the error is an exim error.
Any misconfiguration on my part?

Regards

Henning