I have recently started experiencing sporadic outgoing email faults on different home computers.
Both computers have been rebooted as well as my router and other connected equipment.

My syslogs show nothing obvious relating to smtp or network issues. My ISP cannot find anything in their logs.

Has anybody else with this router model suffered similar intermittent problems and could it be a sign of router failure?

Thanks.

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Desktop PC, Linux Mint 18 Sarah 64-bit
Desktop PC, Lubuntu 14.04 32-bit
Vigor2860n Router
- Firmware Version: 3.8.2.3_VT1
- Build Date/Time: Apr 22 2016 18:05:20
- DSL Version: 562301_A HW: A

My linux /var/log/syslog shows this...
Jun 30 09:09:48 gjdpc4 kernel: [ 2186.120068] [UFW BLOCK] IN=enp3s0 OUT= MAC= SRC=192.168.1.64 DST=224.0.0.251 LEN=32 TOS=0x00 PREC=0xC0 TTL=1 ID=0 DF PROTO=2

where SRC=192.168.1.64 is my main desktop computer.