SASL LOGIN authentication failed

[@lbutlr]

In these log lines, what is "UGFzc3dvcmQ6"?

May 12 07:52:07 mail submit-tls/smtpd[32670]: warning: 
vps1590651.vs.webtropia-customer.com[62.141.41.104]: SASL LOGIN authentication 
failed: UGFzc3dvcmQ6
May 12 17:05:14 mail submit-tls/smtpd[87898]: warning: 
ma350.mars.fastwebserver.de[193.111.198.88]: SASL LOGIN authentication failed: 
UGFzc3dvcmQ6
May 12 18:21:36 mail submit-tls/smtpd[65165]: warning: 
vps1590646.vs.webtropia-customer.com[62.141.41.114]: SASL LOGIN authentication 
failed: UGFzc3dvcmQ6