SMTP client tests overhaul #81

Job	Run time
Test with Codecov coverage (ubuntu-latest / 1.23)	58s
Go vulnerabilities check	22s
Dependency review	11s
golangci-lint (1.23)	36s
REUSE Compliance Check	17s
Test with SonarQube review (ubuntu-latest / 1.23)	2m 23s
Test on FreeBSD 13.4'	2m 7s
Test on FreeBSD 14.0	2m 11s
Test on FreeBSD 14.1	2m 28s
Test (macos-latest / 1.19)	27s
Test (macos-latest / 1.21)	19s
Test (macos-latest / 1.22)	43s
Test (macos-latest / 1.20)	28s
Test (macos-latest / 1.23)	21s
Test (ubuntu-latest / 1.19)	28s
Test (ubuntu-latest / 1.20)	29s
Test (ubuntu-latest / 1.21)	22s
Test (ubuntu-latest / 1.22)	21s
Test (ubuntu-latest / 1.23)	25s
Test (windows-latest / 1.19)	56s
Test (windows-latest / 1.20)	3m 5s
Test (windows-latest / 1.21)	2m 2s
Test (windows-latest / 1.22)	1m 51s
Test (windows-latest / 1.23)	44s
	24m 34s

Provide feedback