Error in /var/log/exim_mainlog - dropped: too many syntax or protocol errors (last command was ...

Symptoms

A user is unable to send an email and an error similar to one of the following can be found in exim_mainlog.

SMTP call from remotehostname [203.0.113.2]:56856 dropped: too many syntax or protocol errors (last command was "RCPT TO: <'user@domain.tld'>",  C=EHLO,AUTH,MAIL,RCPT,RCPT,RCPT,RCPT,RCPT,RCPT)

SMTP call from remotehostname [203.0.113.2]:37034 dropped: too many syntax or protocol errors (last command was "?", NULL)

Description 

The email client in use sent extra or invalid data with a command. This commonly occurs when a list or script is used to generate the email recipients.

Workaround

Manually enter the recipient address in the email client. Ensure that there are no extraneous characters, such as spaces or quotation marks.