Error from IMAP server: invalid tag <tag>

[david-perez]

There seems to be some error in the way the (at least the first) IMAP commands are being sent to the server by the proxy. If you analyze the network traffic you'll see that the server always replies with a BAD invalid tag <tag> to the first CAPABILITY command, but everything thereafter seems OK.

I suspect the proxy is sending an extra CRLF. If you substitute the POE::Filter::Line->new( Literal => LINESEP) with POE::Filter::Stream (i.e. a no-op filter), the error goes away, confirming that its cause is indeed within the proxy and not due to the mail synchronizer. But of course the program needs to split the lines by CRLF for the label-extracting regexes to work correctly, so the filters are necessary.

I initially discovered this using mbsync -DN for learning purposes (in fact mbsync alone will complain), but it of course also takes place with offlineimap (offlineimap does not complain, you have to inspect the traffic).