[exim] only RSET accepted now

Hi all,

I use the latest Horde installation via pear with PHP7 and the latest
exim 4.88 as an IMAP / SMTP server (debian package). 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 error generated by exim.
I have not set a message size limit, so it should be at 50 MB by default.
I have talked to the programmer of the SMTP code and Imp from the
horde framework and he thinks, that this is an exim bug. the log file
of exim shows nothing.

Does anyone else see something similar?

Regards

Henning