nullmailer failing with gnutls >= 3.6.3 under Debian

Seems like nullmailer is having a problem with the new 3.6.4 release of
gnutls (as well as with 3.6.3 version):

7174 nullmailer-send->gnutls_error_is_fatal(0, 0, 0, 0x560d7cc36460)                                                   = 0
7174 nullmailer-send->gnutls_record_send(0x560d7cc36470, 0x560d7cc46430, 36, 0 <unfinished ...>
7174 libgnutls.so.30->gnutls_record_send2(0x560d7cc36470, 0x560d7cc46430, 36, 0 <unfinished ...>
7174 libgnutls.so.30->gnutls_record_get_max_size(0x560d7cc36470, 0x560d7cc46430, 0x7f0ef7d49e60, 0)                    = 0x4000
7174 libgnutls.so.30->gnutls_record_overhead_size(0x560d7cc36470, 0x560d7cc46430, 0x7f0ef7d49e60, 0 <unfinished ...>
7174 libgnutls.so.30->_gnutls_record_overhead(0x7f0ef7d49e60, 0x7f0ef7d471c0, 0x7f0ef7d49bc0, 1)                       = 17
7174 <... gnutls_record_overhead_size resumed> )                                                                       = 22
7174 libgnutls.so.30->gnutls_aead_cipher_encryptv(0x560d7cc51e20, 0x7ffcecde3f10, 12, 0x7ffcecde3ed0)                  = 0
7174 <... gnutls_record_send2 resumed> )                                                                               = 36
7174 <... gnutls_record_send resumed> )                                                                                = 36
7174 nullmailer-send->gnutls_record_recv(0x560d7cc36470, 0x560d7cc58510, 4096, 0x560d7bce839c <unfinished ...>
7174 libgnutls.so.30->gnutls_aead_cipher_decrypt(0x560d7cc51cf8, 0x7ffcecde3de0, 12, 0x7ffcecde3dd0 <unfinished ...>
7174 libgnutls.so.30->gnutls_memcmp(0x560d7cc461fa, 0x7ffcecde3c70, 16, 0xd598af805c08b3d4)                            = 0
7174 <... gnutls_aead_cipher_decrypt resumed> )                                                                        = 0
7174 libgnutls.so.30->gnutls_buffer_append_data(0x560d7cc36798, 0x560d7cc53914, 53, 53)                                = 0
7174 <... gnutls_record_recv resumed> )                                                                                = -28

As you can see the send works but the recv gets -28 which means
GNUTLS_E_AGAIN:

https://github.com/rsyslog/rsyslog/issues/1840 rsyslog has the same
problem

https://codesearch.debian.net/search?q=gnutls_record_recv+package%3Aneomutt
and this is how neomutt deals with the issue (gnutls/gnutls.h contains
GNUTLS_E_AGAIN)

Could you please take a look at this issue?

Thanks
Harald