I recently tried to send an email to a colleague, and received the following error:

local-part of envelope RCPT address contains utf8 but the remote server did not offer SMTPUTF8

Many posts suggest that you avoid the UTF8 option in Gmail. This option, however, seems no longer to exist in either the Inbox interface or the "old" Gmail interface.

Furthermore, using the plain-text option, doesn't seem to change anything either. This post has tried to raise the same issue in the past but got closed.

Further details – the mail itself contains no attachments, "special" characters or anything that should require out of the way encoding.

Edit: Based on @pnuts comment, it is important to note that the address contains a - symbol. It seems like sometimes this is a problem, but there must be a way around it. I mean, it is this guy's email.