This is related to the size Size restriction for outbound e-mails and happens when you are trying to send an email that is too large.
The SAP system is set up so that only e-mails under a certain size are sent using the SAPconnect SMTP interface because performance problems (TIME_OUT, SYSTEM_NO_ROLL) can occur when large email body parts are processed within the SAPconnect send process. You want to prevent this by applying a size restriction but what if this limit is too small?
SMTP, mail, SCOT, send process, RSCONN01
If you have an email that you need to send but is failing due to a size issue you can try setting the following parameter in the table SXPARAMS.
MAXLEN_MIME_MESSAGE_SMTP 0 … 999999999999
The parameter value determines the maximum size of an e-mail (in bytes) that may be sent using the SAPconnect SMTP interface. A value of 0 means no restriction and can be done via transaction SM30 or within t-code SCOT
You may get a warning, simply press enter
Enter a value for the parameter
Alternatively, maintain via transaction SM30 in the usual way
Also see other parameters which are related to SAP OOS Note 837300
Parameters BTC_FIRSTLINE and BTC_LASTLINE in table TSPOPTIONS
Also check out profile parameter mpi/total_size_MB. If you have tried all the above have a look at this and even if it already seems like it is set high enough try setting it higher just to make sure this is not the issue I would recommend trying between 500 and 1000.