While setting up Mailgun with a domain who's DNS records are setup inside AWS's Route 53, I received the following error:
CharacterStringTooLong (Value is too long) encountered with '"k=rsa; ...more..."'
This was in response to creating a TXT record (DKIM) which hs a very long value. 400 characters in fact.
The reason is that the TXT record is limited to 255 characters but you can have multiple strings in the record.
So, the way to enter the long value is to split the string up into pieces which are no longer than 255 characters.
Then swap the sub-strings so they read last as first.
"PARTA" "PARTB" is entered as: "PARTB" "PARTA"