Hello Denis,
If you are using GWI (Groupware Integration with client-client integration), then check the following documentation:
Basically, the transaction types can be different for inbound and outbound emails. In IMG (transaction SPRO) simply follow the path Customer Relationship Management -> Basic Functions -> One-to-One Email -> Assign Transaction Types. Default process types are 0004 and 0005.
Hope this helps.
Regards,
Dawood.