当我尝试使用 1and1 的 SMTP 服务器通过 swiftmailer 发送邮件时，遇到了长时间的延迟。每次都有 19 秒的延迟，如 tcpdump 所示：
21:54:50.784747 IP (tos 0x0, ttl 64, id 40875, offset 0, flags [DF], proto TCP (6), length 94)
my-website.co.uk.46086 > mrvnet.kundenserver.de.smtp: Flags [P.], cksum 0xc764 (incorrect -> 0x1ba9), seq 65:107, ack 193, win 115, options [nop,nop,TS val 3384859388 ecr 1663195506], length 42
E..^..@.@.........~......`Y.+.E8...s.d.....
....c"YrRCPT TO: <[email protected]>
21:54:50.825354 IP (tos 0x0, ttl 60, id 4346, offset 0, flags [DF], proto TCP (6), length 52)
mrvnet.kundenserver.de.smtp > my-website.co.uk.46086: Flags [.], cksum 0x1c51 (correct), seq 193, ack 107, win 12, options [nop,nop,TS val 1663195516 ecr 3384859388], length 0
E..4..@.<.f...~.........+.E8.`Y6.....Q.....
c"Y|....
21:55:09.786486 IP (tos 0x0, ttl 60, id 4347, offset 0, flags [DF], proto TCP (6), length 66)
mrvnet.kundenserver.de.smtp > my-website.co.uk.46086: Flags [P.], cksum 0x1fb6 (correct), seq 193:207, ack 107, win 12, options [nop,nop,TS val 1663200256 ecr 3384859388], length 14
E..B..@.<.f...~.........+.E8.`Y6...........
c"l.....250 Accepted
有人知道为什么 SMTP 此时可能会出现延迟吗？