This issue has been resolved in Open Transport 1.0.8 and later, so you should upgrade your version of Open Transport. Version 1.1 is part of System 7.5 Update 2.0, which is available from the online services where Apple posts software updates. Also, Qualcomm currently recommends using Eudora versions 1.5.3, 1.5.4, 2.1.3, 2.1.4 (or newer) with Open Transport.
If you do not have access to Open Transport 1.0.8 or later the workaround below may work.
Eudora 1.5.1 does work properly with Open Transport unless you are using a SLIP or PPP connection. In this case, Eudora has a issue sending small mail messages.
The workaround at this point is to make sure the mail message contains at least 1500 characters. This seems to allow the message to be sent every time. This workaround has not received extensive testing and is not guaranteed to work. In fact, 1500 characters may be a bit of overkill but it seems to be reliable.
This article can help you locate the software update mentioned here: